LAMMPS Input Commands

Return to top-level of LAMMPS documentation.

This page contains a complete list of valid LAMMPS commands which are read-in from an input script. It will be easiest to understand if you read it while looking at sample input scripts in the examples directory.

The script of input commands is read by LAMMPS, one line at a time. Each command causes LAMMPS to take some action. Usually it simply causes some internal variable(s) to be set. Or it may cause a data file to be read in or a simulation to be run. Note that most commands have default settings, which means you only need use a particular command if you do not want the default setting.

Each LAMMPS input script contains exactly one "read data" (or "read restart") command which defines the problem to be simulated. All other commands can be split into three categories: (a) commands that (if used) must appear before the "read data" command because they define settings needed to correctly read-in the problem and allocate memory for it, (b) commands that must appear after the "read data" command because they act on the specified problem, and (c) commands that can appear either before or after the "read data" command. Commands in category (c) are used before the "read data" command if a default setting needs to be changed before the problem description is read-in. They can be used after the "read data" command if the user wishes to change a setting before the next "run" or "minimize" command is used. Other than these restrictions, commands can generally appear in any order in the input script, although some commands require others to have been previously specified.

Each LAMMPS input script also contains one or more "run" or "minimize" commands. These trigger an actual dynamics or minimization computation to be done. Following a run, new commands from categories (b) and (c) can be used to change various settings, and additional "run" commands can then be used to continue the previous simulation. LAMMPS continues to read successive lines from the input script until the end-of-file is reached, which causes LAMMPS to terminate.

This page gives examples of each command, some of which can be specified in multiple styles. Typically the commands take one or more parameters. The keyword for each command should begin in the leftmost column and all characters in the command and its parameters should be in lower-case (except the word NULL or characters in filenames). Parameters can be separated by arbitrary numbers of spaces and/or tabs, so long as the command fits on one line. The remainder of the line after the last parameter is ignored.

The next section outlines the structure of a LAMMPS input script. The final section gives a detailed description of the commands in alphabetic order, each with its associated parameters and default settings.


Structure of a LAMMPS input script

Any line starting with a # is a comment. Comments can appear anywhere in the input script.

(1) Initialization settings (must appear before "read data" or "read restart")

(2) Optional Settings (can appear before and/or after "read data" or "read restart")

(3) Read in a Problem via a "read data" or "read restart" command

(4) Optional Settings (same as (2))

(5) Problem Settings (must appear after "read data" or "read restart")

(6) Perform a Simulation via a "run" or "minimize" command

Repeat (4), (5), and (6) as desired ...


Initialization Settings

(if used, must appear before "read data" or "read restart" command)
units                  real

extra memory           2.0 1.5 2.0 2.5
dimension              3
processor grid         10 10 10
periodicity            0 0 0
slab volume            3.0
newton flag            3
true flag              0
maximum cutoff         10.0
mixing style           geometric
restart version        5

Optional Settings

(if used, can appear before and/or after "read data" or "read restart" command)
neighbor               2.0 1 1 10 1

nonbond style          none
nonbond style          lj/cutoff 10.0 0
nonbond style          lj/smooth 8.0 10.0
nonbond style          lj/shift 10.0 0
nonbond style          soft 2.5
nonbond style          class2/cutoff 10.0 0
nonbond style          lj/charmm 15.0 15.1
nonbond style          expo_6/cutoff 10.0 0

coulomb style          none
coulomb style          cutoff 10.0
coulomb style          smooth 8.0 10.0
coulomb style          ewald 10.0 1.0E-4
coulomb style          pppm 10.0 1.0E-4
coulomb style          charmm/switch 15.0 15.1

bond style             none
bond style             harmonic
bond style             fene/standard
bond style             fene/shift
bond style             nonlinear
bond style             class2

angle style            none
angle style            harmonic
angle style            class2
angle style            charmm

dihedral style         none
dihedral style         harmonic
dihedral style         mutliharmonic
dihedral style         class2
dihedral style         charmm

improper style         none
improper style         harmonic
improper style         cvff
improper style         class2
inversion style         none
inversion style         umbrella

Read in a Problem

read data              data.lj
read restart           restart.100000

Velocity Creation

(if used, must appear after "read data" or "read restart" command)
create group           types 1 3
create group           molecules 200 300
create group           region 0.0 1.0 0.0 1.0 INF 1.0
create group           remainder

rotation zero          1

create temp            uniform 300.0 12345678
create temp            gaussian 300.0 12345678
create temp            velocity 0.0 0.0 0.0

Force Field Parameters

(if used, must appear after "read data" or "read restart" command)
nonbond coeff          1 2 1.0 3.45 10.0         (nonbond style lj/cutoff)
nonbond coeff          1 2 1.0 3.45 8.0 10.0     (nonbond style lj/smooth)
nonbond coeff          1 2 1.0 3.45 2.0 10.0     (nonbond style lj/shift)
nonbond coeff          1 2 1.0 30.0 2.5          (nonbond style soft)
nonbond coeff          1 2 1.0 3.45 10.0         (nonbond style class2/cutoff)
nonbond coeff          1 2 1.0 3.45 1.0 3.45     (nonbond style lj/charmm)

special bonds          amber
special bonds          0.0 0.0 0.5

pppm mesh              32 32 64
pppm order             5
dielectric             1.0

bond coeff             1 100.0 3.45              (bond style harmonic)
bond coeff             1 30.0 1.5 1.0 1.0        (bond style fene/standard )
bond coeff             1 30.0 1.5 1.0 1.0 0.2    (bond style fene/shift)
bond coeff             1 28.0 0.748308 0.166667  (bond style nonlinear)
angle coeff            1 30.0 108.0              (angle style harmonic)
angle coeff            1 30.0 108.0 30.0 2.5     (angle style charmm)
dihedral coeff         1 10.0 1 3                (dihedral style harmonic)
dihedral coeff         1 2.0 2.0 2.0 2.0 2.0     (dihedral style multiharmonic)
dihedral coeff         1 2.0 5 180.0 0.5         (dihedral style charmm)
improper coeff         1 20.0 0.0                (improper style harmonic)
improper coeff         1 20.0 10.0               (improper style cvff)

Constraints

(if used, must appear after "read data" or "read restart" command)
fix style              none
fix style              1 setforce 0.0 NULL 0.0
fix style              1 addforce 1.0 0.0 0.0
fix style              1 aveforce 1.0 0.0 0.0
fix style              1 rescale 300.0 300.0 100 20.0 0.5
fix style              1 hoover/drag 50.0 50.0 0.001
fix style              1 langevin 50.0 50.0 0.01 12345 1 1 1
fix style              1 springforce 10.0 NULL NULL 1.0
fix style              1 dragforce 10.0 -5.0 NULL 2.0 1.0
fix style              1 shake 3 0.001 100

assign fix             1 atom 200
assign fix             1 molecule 50
assign fix             1 type 2
assign fix             1 region 0.0 1.0 INF INF 0.0 1.0
assign fix             1 bondtype 4
assign fix             1 remainder

Ensemble Control

(if used, must appear after "read data" or "read restart" command)
temp control           none
temp control           rescale 300.0 300.0 100 20.0 0.5
temp control           replace 300.0 300.0 50 12345678
temp control           langevin 50.0 50.0 0.01 123456
temp control           nose/hoover 300.0 300.0 0.01

press control          none
press control          nose/hoover xyz 0.0 0.0 0.001
press control          nose/hoover xz 0.0 10.0 5.0 5.0 0.0 10.0 0.001
press control          nose/hoover yz NULL NULL 5.0 5.0 0.0 10.0 0.001
press control          nose/hoover aniso 0.0 0.0 0.0 0.0 1.0 10.0 0.001
press control          nose/hoover aniso 0.0 0.0 0.0 0.0 NULL NULL 0.001

volume control         none
volume control         linear x 0.0 10.0
volume control         linear y -1.0 15.0
volume control         linear z -10.0 10.0

Output Control

(if used, must appear after "read data" or "read restart" command)
thermo flag            50
thermo style           0

dump atoms             100 filename
dump trajectory        100 filename
dump velocities        100 filename
dump forces            100 filename

restart                1000 1 filename
restart                1000 2 file1 file2
diagnostic             diffusion 100 filename 3 1.0 -1.0 2.5

Integrator Settings

(if used, must appear after "read data" or "read restart" command)
timestep               1.0
respa                  2 2 4
reset timestep         0

Minimizer Settings

(if used, must appear after "read data" or "read restart" command)
min style              hftn
min flag               10

Perform a Simulation

run                    10000
minimize               0.0001 9999 50000


Alphabetic Listing of Commands:


angle coeff

coeffs:   harmonic
               (1) K (energy units)
               (2) theta (degrees)
          class2
               currently not enabled for "angle coeff" command
               must be specified in data file (see "read data" command)
          charmm
               (1) K (energy units)
               (2) theta (degrees)
               (3) K_UB (energy/distance^2)
               (4) r_UB (distance)

define (or override) angle coefficients for an individual angle type
use appropriate number of coeffs for a particular style
see force_fields.html for meaning of coefficients for each style
these coefficients can also be set in data file 
  by a "Angle Coeffs" entry, the most recently defined 
  coefficients are used
cannot use this command before a "read data" or "read restart" is performed,
  because memory is not yet allocated for the necessary arrays

angle style

define style of angle interactions to use for all 3-body terms
must be used before the "read data" command (if not using the
  default) to tell the program how to read the "Angle Coeffs" entry
  in the data file
can be used after the "read data" command to change the style to none
coefficients for all angle types must be defined in the data (or restart)
  file by a "Angle Coeffs" entry or by "angle coeff"
  commands before a run is performed

Default = harmonic

assign fix

styles:
coeffs:  atom
               (1) global atom #
         molecule
               (1) molecule #
         type
               (1) atom type
         region
               (1) lower x bound of region
               (2) upper x bound of region
               (3) lower y bound of region
               (4) upper y bound of region
               (5) lower z bound of region
               (6) upper z bound of region
         bondtype
               (1) bond type
         remainder
               no other parameters required

assign a group of atoms or a bond type to a particular constraint
use appropriate number of coeffs for a particular style
the constraint itself must first be defined by a 
  "fix style" command
multiple groups of atoms or bond types can be assigned to the same constraint
the bondtype option can only be assigned to a "fix style" of "shake",
  multiple bondtypes can be SHAKEn, so long as the size of clusters of
  atoms does not exceed the limit described in the "fix style" command
an atom can be assigned to multiple constraints, the contraints will be
  applied in the reverse order they are assigned to that atom
  (e.g. each timestep, the last fix assigned to an atom will be applied 
  to it first, then the next-to-last applied second, etc)
for style region, a coeff of INF means + or - infinity (all the way 
  to the boundary)

bond coeff

coeffs:   harmonic
               (1) K (energy units)
               (2) r0 (distance units)
          fene/standard
               (1) k for FENE portion (energy/distance^2 units)
               (2) r0 for FENE portion (distance units)
               (3) epsilon for LJ portion (energy units)
               (4) sigma for LJ portion (distance units)
          fene/shift
               (1) k for FENE (energy/distance^2 units)
               (2) r0 for FENE after shift is performed (distance units)
               (3) epsilon for LJ (energy units)
               (4) sigma for LJ after shift is performed (distance units)
               (5) delta shift distance (distance units)
          nonlinear
               (1) epsilon (energy units)
               (2) r0 (distance units)
               (3) lamda (distance units)
          class2
               currently not enabled for "bond coeff" command
               must be specified in data file (see "read data" command)

define (or override) bond coefficients for an individual bond type
use appropriate number of coeffs for a particular style
see force_fields.html for meaning of coefficients for each style
these coefficients can also be set in data file 
  by a "Bond Coeffs" entry, the most recently defined 
  coefficients are used
cannot use this command before a "read data" or "read restart" is performed,
  because memory is not yet allocated for the necessary arrays

bond style

define style of bond interactions to use between all bonded atoms
must be used before the "read data" command (if not using the
  default) to tell the program how to read the "Bond
  Coeffs" entry in the data file (if one exists)
can be used after the "read data" command to change the style,
  in this case "bond coeff" commands must also be used to set new
  coefficients for each bond type (unless the new style is "none")
coefficients for all bond types must be defined in the data (or restart)
  file by a "Bond Coeffs" entry or by "bond coeff"
  commands before a run is performed

Default = harmonic

comments

blank lines are ignored
lines starting with a # are echoed into the log file
for commands, everything on a line after the last parameter is ignored

coulomb style

styles:
coeffs:  none
               no other parameters required
         cutoff
               (1) cutoff distance (distance units)
         smooth
               (1) inner cutoff (distance units)
               (2) outer cutoff (distance units)
         ewald
               (1) cutoff distance for near-field portion (distance units)
               (2) accuracy criterion
         pppm
               (1) cutoff distance for near-field portion (distance units)
               (2) accuracy criterion
         charmm/switch
               (1) inner cutoff (distance units)
               (2) outer cutoff (distance units)

use appropriate number of coeffs for a particular style
normally this command should be used before "read data" or "read restart"
  (if simulating a charged system) to tell LAMMPS how big a force cutoff
  is being used, the "maximum cutoff" command can also serve this
  purpose
restart files do not store "coulomb style" choice or cutoff, so
  this should be specified in the input script when running from a restart
  file
this command can also be used after "read data" or "read restart" to
  change the style of Coulomb interactions or the cutoff
if simulated system has no charges, should set "coulomb style none" to
  prevent LAMMPS from doing useless nonbond work, LAMMPS will set
  this for you and issue a warning
cutoff distance can be smaller or larger than simulation box dimensions
accuracy criterion means "one part in value" - e.g. 1.0E-4
Ewald and PPPM accuracy criterion are used in conjunction with cutoff
  to partition work between short-range and long-range routines
accuracy criterion effectively determines how many k-space vectors are used
  to approximate the energy and forces
for PPPM, accuracy criterion determines mesh spacing (see "particle mesh"
  command)
3-d periodic boundary conditions are normally used in conjunction with
  Ewald and PPPM, see "slab volume" command for 2-d Ewald/PPPM
cannot use any Coulomb styles other than none with nonbond style = lj/shift or
  nonbond style = soft
Coulomb style = smooth should be used with nonbond style = lj/switch,
  and both should use same inner and outer cutoffs
nonbond style = lj/charmm should be used with coulomb style = charmm/switch
for smooth and charmm/switch styles, outer cutoff must be > inner cutoff
for smooth and charmm/switch styles, atom pairs less than the inner cutoff
  distance use usual Coulomb, pairs between inner and outer are smoothed,
  and the potential goes to 0.0 at the outer cutoff
for smooth style, force is continuously differentiable everywhere

Default = cutoff 10.0   for real units
          cutoff 2.5    for lj units

create group

styles:
coeffs:  types
               (1) lowest atom type
               (2) highest atom type
         molecules
               (1) lowest molecule ID
               (2) highest molecule ID
         region
               (1) lower x bound of region
               (2) upper x bound of region
               (3) lower y bound of region
               (4) upper y bound of region
               (5) lower z bound of region
               (6) upper z bound of region
         remainder
               no other parameters required
       
used with "create temp" commmand to initialize velocities of atoms
by default, the "create temp" command initializes the velocities of all atoms,
  this command limits the initialization to a group of atoms
this command is only in force for the next "create temp" command, any
  subsequent "create temp" command is applied to all atoms (unless the
  "create group" command is used again)
for style types, only atoms with a type such that lo-type <= type <= hi-type
  will be initialized by "create temp"
for style types, lo-type can equal hi-type if just want to specify one type
for style molecules, only atoms belonging to molecules with an ID # such 
  that lo-ID <= type <= hi-ID will be initialized by "create temp"
for style molecules, lo-ID can equal hi-ID if just want to specify one molecule
for style region, only atoms within the specified spatial region
  will be initialized by "create temp"
for style region, a coeff of INF means + or - infinity (all the way 
  to the boundary)
for style remainder, only previously uninitialized atoms
  will be initialized by "create temp"

create temp

styles:
coeffs:  uniform
               (1) target T (temperature units)
               (2) random # seed (0 < seed <= 8 digits)
         gaussian
               (1) target T (temperature units)
               (2) random # seed (0 < seed <= 8 digits)
         velocity
               (1) x velocity component (velocity units)
               (2) y velocity component (velocity units)
               (3) z velocity component (velocity units)

initialize velocities of atoms to a specified temperature
use appropriate number of coeffs for a particular style
cannot be done before a data or restart file is read
by default, velocities are created for all atoms - this can be overridden
  by first using a "create group" command
for uniform and Gaussian styles velocities are created in 
  processor-independent fashion - is slower but gives the same initial 
  state independent of # of processors
for uniform and Gaussian styles the momentum of the initialized atoms is
  also zeroed, but only if all atoms are being initialized 
for uniform and Gaussian styles, RN are generated with Park/Miller RNG
for velocity style in 2-d simulations, still specify z velocity component,
  even though it is ignored

diagnostic

call a user-defined diagnostic routine every this many timesteps
this command can be used multiple times to call different routines
  at different frequencies, that use different parameters, and that
  send output to different files
value of 0 for 2nd parameter means never call this particular routine
this command causes any previous file associated with this user routine
  to be closed
new filename can exist, will be overwritten
if the file name specified is "none", then no file is opened
each routine that is added to diagnostic.f and enabled with a
  "diagnostic" command will be called at the beginning and end of
  each "run" and every so many timesteps during the run
see *** comments in diagnostic.f for changes that must be made in 
  that file to enable user diagnostics, LAMMPS must then be re-compiled
  and re-linked
see the diagnostic.f file for further information on how to create
  routines that operate on internal LAMMPS data, do their own file output,
  perform different operations (e.g. setup and clean-up) depending
  on when they are called, etc
the optional 5th-9th parameters are stored as internal LAMMPS variables
  which can be accessed by the diagnostic routine

dielectric

set dielectric constant to this value

Default = 1.0

dihedral coeff

coeffs:   harmonic
               (1) K (energy units)
               (2) d (+1 or -1)
               (3) n (1,2,3,4,6)
          multiharmonic
               (1) A_1 (energy units)
               (2) A_2 (energy units)
               (3) A_3 (energy units)
               (4) A_4 (energy units)
               (5) A_5 (energy units)
          class2
               currently not enabled for "dihedral coeff" command
               must be specified in data file (see "read data" command)
          charmm
               (1) K (energy units)
               (2) n (1,2,3,4,6)
               (3) d (0 or 180 degrees) (converted to radians within LAMMPS)
               (4) weighting factor to turn on/off 1-4 neighbor nonbond interactions

define (or override) dihedral coefficients for an individual dihedral type
use appropriate number of coeffs for a particular style
see force_fields.html for meaning of coefficients for each style
these coefficients can also be set in data file 
  by a "Dihedral Coeffs" entry, the most recently defined 
  coefficients are used
cannot use this command before a "read data" or "read restart" is performed,
  because memory is not yet allocated for the necessary arrays

dihedral style

define style of dihedral interactions to use for all 4-body terms
must be used before the "read data" command (if not using the
  default) to tell the program how to read the "Dihedral
  Coeffs" entry in the data file
can be used after the "read data" command to change the style to none
coefficients for all dihedral types must be defined in the data (or restart)
  file by a "Dihedral Coeffs" entry or by "dihedral coeff"
  commands before a run is performed

Default = harmonic

dimension

for a 2-d run, assumes all z-coords are set to 0.0 in "read data" or
  "read restart" files and program creates no z velocities
this command sets the processor grid to default values for 2-d or 3-d
  so must be used before "processor grid" command
must be set before data or restart file is read

Default = 3

dump atoms

dump all atom positions to a file every this many timesteps
  (every this many iteration when the minimizer is invoked)
when rRESPA is enabled, this is steps of outermost loop (longest timesteps)
positions are also dumped at the start and end of each run
when dumped during minimization, all dumps will have the same timestamp
  since the timestep does not change during minimization
value of 0 means never dump
any previous file is closed
new filename can exist, will be overwritten
atom positions in dump file are in "box" units (0.0 to 1.0) in each dimension
IMPORTANT NOTE: due to the way periodic boundary conditions are enforced 
  (only when neighbor lists are rebuilt), atom coords appearing in the dump
  file can be slightly outside the specified box

Default = 0

dump trajectory

dump all atom positions and velocities to a binary file every this many timesteps
when rRESPA is enabled, this is steps of outermost loop (longest timesteps).
The trajectory is also dumped at the start and end of each run.
Value of 0 means never dump
any previous file is closed
new filename can exist, will be overwritten
atom positions in trajectory file are in input coordinate units 
IMPORTANT NOTE: due to the way periodic boundary conditions are enforced
  (only when neighbor lists are rebuilt), atom coords appearing in the trajectory
  file can be outside the specified box

Default = 0

dump forces

dump all atom forces to a file every this many timesteps
  (every this many iteration when the minimizer is invoked)
when rRESPA is enabled, this is steps of outermost loop (longest timesteps)
forces are also dumped at the start and end of each run
when dumped during minimization, all dumps will have the same timestamp
  since the timestep does not change during minimization
any previous file is closed
new filename can exist, will be overwritten
value of 0 means never dump

Default = 0

dump velocities

dump all atom velocities to a file every this many timesteps
when rRESPA is enabled, this is steps of outermost loop (longest timesteps)
velocities are also dumped at the start and end of every run
any previous file is closed
new filename can exist, will be overwritten
value of 0 means never dump

Default = 0

extra memory

factors that affect how much extra memory is allocated when a problem is setup
factor of 1.0 means no padding (use exactly what LAMMPS estimates is
  needed), factor of 2.0 means 2x longer arrays, etc
typically don't need to change default settings unless LAMMPS tells you
  to "boost" some factor at run-time
final section of log file lists optimal settings for these parameters,
  i.e. the job could have been run with those "extra memory" settings
  and would have used minimal memory
must be set before data or restart file is read

Default = 1.5 for all 4 parameters

fix style

styles:
coeffs:  none
               no other parameters required (use "none" as 1st parameter)
         setforce
                (1) x component of set force (in force units)
                (2) y component of set force (in force units)
                (3) z component of set force (in force units)
         addforce
                (1) x component of added force (in force units)
                (2) y component of added force (in force units)
                (3) z component of added force (in force units)
         aveforce
                (1) x comp of added average force per atom (in force units)
                (2) y comp of added average force per atom (in force units)
                (3) z comp of added average force per atom (in force units)
         rescale
                (1) desired T at beginning of run
                (2) desired T at end of run
                (3) check for rescaling every this many timesteps
                (4) T window outside of which velocities will be rescaled
                (5) fractional amount (0.0 to 1.0) of rescaling to perform
         hoover/drag
                (1) desired T at beginning of run
                (2) desired T at end of run
                (3) damping constant for drag (roughly inverse time units)
         langevin
                (1) desired T at beginning of run
                (2) desired T at end of run
                (3) Langevin damping parameter (inverse time units)
                (4) random seed to use for white noise (0 < seed <= 8 digits)
                (5) 0/1 = off/on x dimension
                (6) 0/1 = off/on y dimension
                (7) 0/1 = off/on z dimension
         springforce
                (1) x position of spring origin
                (2) y position
                (3) z position
                (4) force constant k (so that k*distance = force units)
         dragforce
                (1) x position to drag atom towards
                (2) y position
                (3) z position
                (4) force magnitude f (in force units)
                (5) delta outside of which to apply force (in distance units)
         shake
                (1) max # of SHAKE iterations within each atom cluster
                (2) SHAKE tolerance (accuracy of one part in tolerance)
                (3) print bond statistics every this many steps (0 = never)

define a constraint
cannot skip a constraint number, all must be used before a run is performed
use appropriate number of coeffs for a particular style
which atoms or bonds the constraint will affect is set by the
  "assign fix" command
all of the constraints (except for rescale) are applied every timestep
all specified temperatures are in temperature units
for style setforce, a coeff of NULL means do not alter that force component
for style aveforce, average force on the group of fixed atoms is computed,
  then new average force is added in and actual force on each atom is set
  to new total value -> has effect of applying same force to entire group
  of atoms
thermostatting constraints (rescale, hoover/drag, langevin) cannot be used in
  conjuction with global "temp control", since they conflict and will
  cause atom velocities to be reset twice
thermostatting constraints (rescale, hoover/drag, langevin) cannot be used
  when performing a minimization
if multiple Langevin constraints are specified the Marsaglia RNG will
  only use the last RNG seed specified for initialization
meaning of rescale and Langevin thermostatting coefficients is same as in 
  "temp control" command
for rescale style, it can be used as a coarse temperature rescaler,
  for example "rescale 200.0 300.0 100 10.0 1.0" will ramp the temperature
  up during the simulation, resetting it to the target temperatue as needed
for rescale style, it can be used to create an instantaneous
  drag force that slowly rescales the temperature without oscillation,
  for example "rescale 300.0 300.0 1 0.0 0.0001" will force (or keep) 
  the temperature to be 300.0, the time frame over which this occurs
  will become longer as the last parameter is made smaller
for hoover/drag style, the drag force accumulates over time so some
  oscillation in temperature can occur, for example
  "rescale 300.0 300.0 1 0.0 0.0001" will force (or keep)
  the temperature to be 300.0, the time frame over which the oscillations
  occur will become longer as the last parameter is made smaller
style springforce is designed to be applied to an entire group of atoms
  en masse (e.g. an umbrella force on an entire molecule)
for springforce style, the center of mass r0 of the group of atoms is computed,
  then a restoring force = -k*(r-r0)*mass/masstotal is applied to each 
  atom in the group where mass = mass of the atom and masstotal = mass of
  all the atoms in the group - thus "k" should represent the total
  force on the group of atoms (not per atom)
for springforce style, a xyz position of NULL means do not include that
  dimension in the distance or force computation
for dragforce style, apply a drag force of magnitude f to each atom in the
  group in the direction (r-r0) where r0 = (x,y,z) - do not apply the force if
  the atom is within a distance delta of r0
for dragforce style, a xyz position of NULL means do not include that
  dimension in the distance or force computation
for shake style, certain bonds in the system are constrained every timestep
  to be at their equilibrium length, this is done by applying a SHAKE-like
  constraint to the forces on the atoms so that their position at the next
  timestep will preserve the atom separations
for shake style, only atoms in small clusters can be constrained -
  e.g. water molecules, CH3 groups, but not the C backbone of a
  long polymer chain - a cluster is defined as a central atom
  connected to others in the cluster by constrained bonds connected
  together by constrained bonds - the max size of such a cluster is
  4 atoms to enable easier parallelization
for shake style, the max iteration count need not be large (e.g. 3) since
  iterations are only done within a cluster and converge quickly
see the "minimize" command for what constraints are allowed for use
  with the minimizer
see the "respa" command for how constraints are applied when rRESPA
  timestepping is enabled

Default = none

improper coeff

coeffs:   harmonic
               (1) K (energy units)
               (2) chi (degrees)
          cvff
               (1) K (energy units)
               (2) d (+1 or -1)
               (3) n (0,1,2,3,4,6)
          class2
               currently not enabled for "improper coeff" command
               must be specified in data file (see "read data" command)

define (or override) improper coefficients for an individual improper type
use appropriate number of coeffs for a particular style
see force_fields.html for meaning of coefficients for each style
these coefficients can also be set in data file 
  by a "Improper Coeffs" entry, the most recently defined 
  coefficients are used
cannot use this command before a "read data" or "read restart" is performed,
  because memory is not yet allocated for the necessary arrays

improper style

define style of improper interactions to use for all trigonal centers
in class2 case, dictates that angle-angle terms be included for all
  trigonal and tetrahedral centers
angle for harmonic is improper torsion, angle for class2 is Wilson out-of-plane
must be used before the "read data" command (if not using the
  default) to tell the program how to read the "Improper
  Coeffs" entry in the data file
can be used after the "read data" command to change the style to none
coefficients for all improper types must be defined in the data (or restart)
  file by a "Improper Coeffs" entry or by "improper coeff"
  commands before a run is performed

Default = harmonic

maximum cutoff

specifies the longest force cutoff that will be used in any runs
this value is used by LAMMPS to accurately allocate memory
  for neighbor arrays
if the value is inaccurate (e.g. the command is not used), it is not an
  error, but LAMMPS may allocate insufficient memory for neighbor lists
this command is not typically needed if the "nonbond style" and "coulomb style"
  commands are used before the "read data" or "read restart" command, since
  they specify the appropriate cutoffs
an exception to this is if a short cutoff is used initially,
  but a longer cutoff will be used for a subsequent run (in the same
  input script), in this case the "maximum cutoff" command should be
  used to insure enough memory is allocated for the later run
note that a restart file contains nonbond cutoffs (so it is not necessary
  to use a "nonbond style" command before "read restart"), but LAMMPS
  still needs to know what the maximum cutoff will be before the
  restart file is read
must be set before data or restart file is read

Default = cutoffs for nonbond and Coulomb styles

min flag

write out minimization info every this many iterations
value of 0 means never write

Default = 1

min style

choose minimization algorithm to use when "minimize" command is performed
currently, the hftn style is the only option available

Default = hftn

minimize

perform an energy minimization of the atomic coordinates of the system
uses algorithm selected with "min style" command
minimize commands can be interspersed with "run" commands
  to alternate between dynamics and relaxation of the system
minimization stops if any of 3 criteria are met:
  (1) largest force component < stopping tolerance
  (2) # of iterations > max iterations
  (3) # of force and energy evaluations > max evaluations
output from the minimizer is specified by the "dump atoms", "dump forces",
  and "restart" commands
when using constraints with the minimizer, fixes are
  applied when atoms move except for the following
fixes associated with temperature control are not allowed
  (rescale, hoover/drag, langevin)
the minimizer does not invoke the "fix style shake" contraints on
  bond lengths
the minimizer does not invoke pressure control or volume control settings
for good convergence, should specify use of smooth nonbond force fields 
  that have continuous second derivatives, e.g. set "coulomb style" to
  "smooth" or "pppm", set "nonbond style" to "lj/smooth" or 
  use a long cutoff

mixing style

styles:
determine the kind of mixing rule that is applied to generate nonbond
  coefficients for interactions between type i and type j atoms
mixing rules are applied only when nonbond coeffs are input in a "read data" file
for nonbond style "soft", only epsilons (prefactor A) are input - they are
  always mixed geometrically, regardless of mixing style setting
for nonbond style "lj/charmm", mixing style is always arithmetic,
  regardless of mixing style setting
must be set before data file is read

Default = geometric for all nonbond styles except
          arithmetic for nonbond style lj/charmm
          sixthpower for nonbond style class2/cutoff

neighbor

factors that affect how and when neighbor lists are constructed
the binning style is almost always faster than the N^2 style
skin must be large enough that all atoms needed for bond
  interactions are also acquired by interprocessor communication
last parameter incurs extra checking and communication to test against
  skin thickness, but may mean neighbor list is created less often
when rRESPA is run, the 3rd and 4th parameters refer to the
  nonbond (short-range) timestepping
normally this command should be used before the data or restart file is read,
  since the skin distance is used to estimate memory needed for
  neighbor lists
this command can also be used after the "read data" or "read restart" command
  to change the style of neighbor list construction, but if the
  skin distance is changed it can cause LAMMPS to run out of neighbor
  list memory, the "maximum cutoff" command can be used to avoid this
  problem

Default = 2.0 1 1 10 1   for real units
          0.3 1 1 10 1   for lj units

newton flag

turn off or on Newton's 3rd law for bond and non-bond force computation
no Newton's 3rd law means more force computation and less communication
yes Newton's 3rd law means less force computation and more communication
which choice is faster is problem dependent on N, # of processors, 
  and cutoff length(s)
expect for round-off errors, setting this flag should not affect answers, 
  only run time
must be set before data or restart file is read

Default = 3

nonbond coeff

coeffs: lj/cutoff
               (1) epsilon (energy units)
               (2) sigma (distance units)
               (3) cutoff (distance units)
        lj/smooth
               (1) epsilon (energy units)
               (2) sigma (distance units)
               (3) inner cutoff (distance units)
               (4) outer cutoff (distance units)
        lj/shift
               (1) epsilon (energy units)
               (2) sigma (distance units)
               (3) delta shift distance (distance units)
               (4) cutoff (distance units)
        soft
               (1) prefactor A at start of run (energy units)
               (2) prefactor A at end of run (energy units)
               (3) cutoff (distance units)
        class2/cutoff
               (1) epsilon (energy units)
               (2) sigma (distance units)
               (3) cutoff (distance units)
        lj/charmm
               (1) epsilon (energy units)
               (2) sigma (distance units)
               (3) epsilon for 1-4 interactions (energy units)
               (4) sigma for 1-4 interactions (distance units)
        expo_6/cutoff
               (1) Ro (distance units)
               (2) Do (energy units)
               (3) y   dimensionless scale factor 

define (or override) nonbond coefficients for an individual atom type pair
use appropriate number of coeffs for a particular style
1st atom type must be <= 2nd atom type
all cutoffs are in global units, not local sigma units
  (e.g. in reduced units a setting of "lj/cutoff 1.0 1.2 2.5" means a 
  cutoff of 2.5, not 1.2*2.5)
turn off a particular type pair interaction by setting the
  cutoff to 0.0 (both cutoffs to zero for lj/smooth option)
for soft style, prefactor A is ramped from starting value to
  ending value during run
these coefficients (except the cutoffs) can also be set in data file 
  by a "Nonbond Coeffs" entry and associated mixing rules, the cutoffs can
  be set (globally) via the "nonbond style" command, the most
  recently defined coefficients/cutoffs are used
cannot use this command before a "read data" or "read restart" is performed,
  because memory is not yet allocated for the necessary arrays
for exponential-6   y>6 is a scale factor. y=12 resembles lj potential

nonbond style

styles:
coeffs:  none
               no other parameters required
         lj/cutoff
               (1) cutoff (distance units)
               (2) offset flag (0 or 1)
         lj/smooth
               (1) inner cutoff (distance units)
               (2) outer cutoff (distance units)
         lj/shift
               (1) cutoff (distance units)
               (2) offset flag (0 or 1)
         soft
               (1) cutoff (distance units)
         class2/cutoff
               (1) cutoff (distance units)
               (2) offset flag (0 or 1)
         lj/charmm
               (1) inner cutoff (distance units)
               (2) outer cutoff (distance units)
         expo_6/cutoff
               (1) cutoff (distance units)
               (2) offset flag (0 or 1)


define style of pairwise nonbond interactions to use between all atom types
use appropriate number of coeffs for a particular style
this is separate from charge interactions (see "coulomb style" command)
normally this command should be used before "read data"
  to tell LAMMPS how big a force cutoff is being used, the 
  "maximum cutoff" command can also serve this purpose
when running from a restart file, the restart file contains the nonbond
  style and nonbond cutoffs (but not the offset flag), so it is often
  not necessary to use a "nonbond style" command before "read restart",
  however LAMMPS still needs to know what the maximum cutoff will be
  before the restart file is read, see "maximum cutoff" command
  for more details
this command can also be used after "read data" or "read restart" to
  change the style of nonbond interactions and/or the cutoff
cutoff distance can be smaller or larger than simulation box dimensions
nonbond style determines how many nonbond coefficients the program expects to
  find in a "Nonbond Coeffs" entry in the data file or when using the 
  "nonbond coeff" command, thus the style must be set (if not using default)
  before using the "read data" command (if the data file contains a 
  "Nonbond Coeffs" entry) or a "nonbond coeff" command
coefficients for all atom type pairs must be defined in data (or restart)
  file by a "Nonbond Coeffs" entry or by "nonbond coeffs" commands before
  a run is performed
this command sets the cutoff(s) for all type pair interactions, thus
  overriding any previous settings by a "nonbond coeff" command or
  that were read in from a data or restart file
for lj/cutoff, lj/shift, class2/cutoff, expo_6/cutoff styles, 
  offset flag only affects printout of thermodynamic energy
  (not forces or dynamics), determines whether offset energy 
  is added in to LJ potential to make value at cutoff = 0.0,
  flag = 0 -> do not add in offset energy,
  flag = 1 -> add in offset energy 
for expo_6/cutoff the A*exp(-CR) - B/r^6 nonbond function is used
  instead of the lennard-jones function.  Currently only a cutoff version has
  been implemented.  Input parameters are in terms of Ro, Do, and y scale factor
for lj/smooth and lj/charmm styles, outer cutoff must be > inner cutoff
for lj/smooth and lj/charmm styles, atom pairs less than the inner cutoff
  distance use straight LJ, pairs between inner and outer use a smoothed LJ,
   and the potential goes to 0.0 at the outer cutoff
for lj/smooth and lj/charmm styles, energy and forces are continuous at inner
   cutoff and go smoothly to zero at outer cutoff
for lj/shift and soft styles, must set "coulomb style" to "none"
for lj/charmm style, must set "coulomb style" to "charmm/switch", "pppm",
   or "ewald"
for lj/shift style, delta shift distances for each atom pair are set by
  "Nonbond Coeffs" entry in data file or by "nonbond coeffs" command
for soft style, values of the prefactor "A", which is ramped from one
  value to another during the run, are set by "Nonbond Coeffs" entry
  in data file or by "nonbond coeffs" command


Default = lj/cutoff 10.0 0   for real units
          lj/cutoff 2.5 0    for lj units

periodicity

turn on/off periodicity in any of three dimensions
used in inter-particle distance computation and when particles move
  to map (or not map) them back into periodic box
for a 2-d run (see "dimension" command), 3rd parameter must be 
  specified, but doesn't matter if it is 0 or 1
must be set before data or restart file is read

Default = 0 0 0 (periodic in all dimensions)

pppm mesh

specify the mesh size used by "coulomb style pppm"
mesh dimensions that are power-of-two are fastest for FFTs, but any sizes
  can be used that are supported by native machine libraries
this command is optional - if not used, a default
  mesh size will be chosen to satisfy accuracy criterion - if used, the
  specifed mesh size will override the default

pppm order

specify the order of the interpolation function that is used by "coulomb
  style pppm" to map particle charge to the particle mesh
order is roughly equivalent to how many mesh points a point charge
  overlaps onto in each dimension

Default = 5

press control

styles:
coupling:
coeffs:   none
               no other parameters required
          nose/hoover xyz
               (1) desired P at beginning of run
               (2) desired P at end of run
          nose/hoover xy or yz or xz or aniso
               (1) desired Px at beginning of run (or NULL, see below)
               (2) desired Px at end of run
               (3) desired Py at beginning of run
               (4) desired Py at end of run
               (5) desired Pz at beginning of run
               (6) desired Pz at end of run
               (7) frequency constant for volume adjust (inverse time units)

enable constant pressure simulations
all specified pressures are in pressure units
any dimension being varied by pressure control must be periodic
for xyz coupling, all 3 dimensions expand/contract together uniformly
  using total scalar pressure as the driving force
for xy/yz/xz coupling, the 2 specified dimensions expand/contract together
  uniformly using pressure components averaged over those 2 dimensions
  as the driving force, the non-specified dimension will expand/contract
  independently using its pressure component as the driving force
for anisotropic, all 3 dimensions expand/contract independently using
  individual pressure components as the 3 driving forces
in all cases, the simulation box stays rectilinear (not Parinello-Rahman)
for dimensions coupled together, their specified P values should be the same
a non-coupled dimension (e.g. dimension z for xy option or any dimension
  for aniso option) can have 2 NULL values as specified pressures,
  which means apply no pressure control in that dimension (constant volume)
target pressure at intermediate points during a run is a ramped value
  between the beginning and ending pressure(s)
for nose/hoover style, frequency constant is like an inverse "piston" 
  mass which determines how rapidly the pressure fluctuates in response to a
  restoring force, large frequency -> small mass -> rapid fluctations
for nose/hoover style, units of frequency/damping constant are
  inverse time, so a value of 0.001 means relax in a timespan on the 
  order of 1000 fmsec (real units) or 1000 tau (LJ units)
IMPORTANT NOTE: the computation of P in LAMMPS does not include
  a long-range Van der Waals correction, this introduces a known
  error when performing constant P simulations since the correction
  factor changes as the box size varies

Default = none

processor grid

specify 3-d grid of processors to map to physical simulation domain
for 2-d problem, specify N by M by 1 grid
program will choose these values to best map processor grid to physical
  simulation box, only use this command if wish to override program choice
product of 3 parameters must equal total # of processors
must be set before data or restart file is read

Default = none

read data

read the initial atom positions and bond info from the specified file
the format for the data file is specified in the file data_format
if a "Velocities" entry is not in data file, all atom velocities
  are set to 0.0
if a "Coeffs" entry is in data file, the appropriate "style" command
  command must be used first (unless default setting is used) to tell 
  LAMMPS how many coefficients to expect
a "Nonbond Coeffs" entry only contains one set of coefficients for each
  atom type, after being read-in mixing rules are applied to 
  compute the cross-type coefficients, see the "mixing style" command
  and data_format file for more information

read restart

read atom and force-field information from specified file
allows continuation of a previous run
file is binary to enable exact restarts
do not have to restart on same # of processors, but can only do exact
  restarts on same # of processors due to roundoff
when restart file is read, warnings are issued if certain parameters
  in the restart file do not match current settings (e.g. newton flag,
  dimension, periodicity, units) - this usually indicates an error
the restart file stores the "nonbond style" and many-body styles and
  coefficients and cutoffs, so these do not have to be re-specified in the
  input script, unless you want to change them
the restart file does not store "coulomb style" choice or cutoff, so
  this should be re-specified in the input script
the restart file stores the constraint assignments for each atom, but
  not the constraint parameters themselves, so they must still be
  specified with "fix style" commands after the file is read

reset timestep

explicitly reset the timestep to this value
the "read data" and "read restart" commands set the timestep to zero
  and the file value respectively, so this should be done after those commands

respa

factors that affect sub-cycling of force calculations within rRESPA hierarchy
bonded intramolecular forces are calculated every innermost sub-timestep
bonded 3- and 4-body forces are computed every 1st parameter sub-timesteps
short-range nonbond pairwise forces (LJ, Coulombic) are computed every 
  (2nd parameter * 1st parameter) sub-timesteps
long-range (Ewald, PPPM) forces are computed every 
  (3rd parameter * 2nd parameter * 1st parameter) sub-timesteps
the timestepping for all 3 inner loops (bond, 3/4-body, nonbond) is performed
  as sub-cycling within the long-range timestepping loop
the fastest (innermost) timestep size is set by the "timestep" command
when running rRESPA, all input commands that specify numbers of timesteps
  (e.g. run, thermo flag, restart, etc) refer to the outermost loop
  of long-range timestepping
the only exception to this rule is the "neighbor" command, where the timestep
  parameters refer to the short-range (nonbond) timestepping
when using constraints (via the "fix style" and "assign fix" commands)
  with rRESPA, the setforce and aveforce constraints are applied at every
  level of the hierarchy (whenever forces are computed), the other
  constraints are applied only at the short-range (nonbond) level
when using "temp control langevin" with rRESPA, thermostatting is applied
  at the short-range (nonbond) level
rRESPA cannot be used with "fix style shake"
setting all 3 parameters to 1 turns off rRESPA

Default = 1 1 1 (no rRESPA)

restart

create a restart file every this many timesteps
value of 0 means never create one
if the style is 1, restart information will be written to files
  named filename.timestep and no 4th parameter is needed
if the style is 2, restart information will be written alternately to files
  given by the 3rd and 4th parameters, so only 2 restart files ever exist
when the minimizer is invoked this command means create a restart file
  at the end of the minimization with the filename filename.timestep.min
a restart file stores atom and force-field information in binary form
allows program to restart from where it left off (see "read restart" commmand)

Default = 0

restart version

tell LAMMPS that a restart file from an older version of LAMMPS will be read-in
  via a "read restart" command
this command is necessary because older restart files have a different format
valid settings are 2001 (LAMMPS 2001), 2000 (LAMMPS 2000),
  6 (LAMMPS 99) or 5 (LAMMPS 5.0)
restart files from earlier versions of LAMMPS are not readable without
  some source code modifications
restart files are always written out in the current-version format
  regardless of this setting
this must be set before the "read restart" command is executed

Default = current version of code = 2001

rotation zero

zero out angular momentum when creating velocities for a group of atoms
value of 0 means don't zero out, value of 1 means zero it

Default = 0

run

run or continue dynamics for specified # of timesteps
when rRESPA is enabled, this is steps of outermost loop (longest timesteps)
must have performed "read data" or "read restart"
  command first

slab volume

invoke 2-d slab Ewald/PPPM and set extended slab volume via this ratio
2-d slab Ewald/PPPM can be used for a system that is periodic in x-y,
  but not in z
this ratio dampens inter-slab interactions in the z dimension 
  by providing empty volume between slabs and removing
  dipole inter-slab interactions
ratio value is the size of the extended dimension in z divided by
  the actual dimension in z
recommended ratio value is 3.0: larger is inefficient, smaller
  risks unwanted inter-slab interactions
when 2-d slab Ewald/PPPM is used, z-direction periodicity must be
  turned off - e.g. periodicity 0 0 1
when 2-d slab Ewald/PPPM is used, user must prevent particle migration
  beyond initial z-bounds, typically by providing walls
2-d slab Ewald/PPPM can only be used only with electrostatically
  neutral systems
2-d slab Ewald/PPPM can only be used (for the moment) with constant
  volume simulations (no pressure control) - the pressure computation
  (printed as thermodynamic data) does not include any slab correction
  factor or a volume correction for the extended z direction
must be set before data or restart file is read

Default = none (normal 3-D Ewald/PPPM)

special bonds

weighting factors to turn on/off nonbond interactions of atom pairs that
  are "close" in the molecular topology
1-2 neighbors are a pair of atoms connected by a bond
1-3 neighbors are a pair of atoms 2 hops away, etc.
weight values are from 0.0 to 1.0 and are used to multiply the
  energy and force interaction (both Coulombic and LJ) between the 2 atoms
weight of 0.0 means no interaction
weight of 1.0 means full interaction
can either specify a single keyword (charmm, amber) or can give
  3 numeric values
using the charmm keyword means use the CHARMM force field
  settings of 0.0 0.0 0.0, requiring that pair-specific 1-4 interactions
  be read in individually (see "dihedral style charmm" command)
using the amber keyword means use the AMBER force field
  settings of 0.0 0.0 N, where N = 0.5 for Van der Waals 1-4 interactions
  and 1.0/1.2 for Coulombic 1-4 interactions

Default = CHARMM force field values of 0.0 0.0 0.0

temp control

styles:
coeffs:  none
               no other parameters required
         rescale
               (1) desired T at beginning of run
               (2) desired T at end of run
               (3) check for rescaling every this many timesteps
               (4) T window outside of which velocities will be rescaled
               (5) fractional amount (0.0 to 1.0) of rescaling to perform
         replace
               (1) desired T at beginning of run
               (2) desired T at end of run
               (3) do Gaussian replacement every this many timesteps
               (4) random # seed to use for replacement (0 < seed <= 8 digits)
         langevin
               (1) desired T at beginning of run
               (2) desired T at end of run
               (3) Langevin damping parameter (inverse time units)
               (4) random seed to use for white noise (0 < seed <= 8 digits)
         nose/hoover
               (1) desired T at beginning of run
               (2) desired T at end of run
               (3) frequency constant for friction force (inverse time units)

enable constant temperature simulations
use appropriate number of coeffs for a particular style
all specified temperatures are in temperature units
target temperature at intermediate points during run is a ramped value
  between the beginning and ending temperatures
for rescale style, temperature is controlled by explicitly rescaling
  velocities towards the target temperature
for rescale style, rescaling is only done if current temperature is
  beyond the target temperature plus or minus the window value
for rescale style, the amount of rescaling is contfolled by the fractional
  amount (0.0 to 1.0), e.g. a value of 0.5 means set the velocities
  to halfway between the current and target temperature
for rescale style, it can be used as a coarse temperature rescaler,
  for example "rescale 200.0 300.0 100 10.0 1.0" will ramp the temperature
  up during the simulation, resetting it to the target temperatue as needed
for rescale style, it can be used to create an instantaneous
  drag force that slowly rescales the temperature without oscillation,
  for example "rescale 300.0 300.0 1 0.0 0.0001" will force (or keep) 
  the temperature to be 300.0, the time frame over which this occurs
  will become longer as the last parameter is made smaller
for replace style, Gaussian RNs from the Marsaglia RNG are used
for langevin style, uniform RNs from the Marsaglia RNG are used
for replace and langevin styles, the seed is used to initialize the
  Marsaglia RNG, on successive runs the RNG will just continue on
for replace and langevin styles, generated RNs depend on # of processors
  so will not get same answers independent of # of processors
for replace and langevin styles, RNG states are not saved in restart file,
  so cannot do an exact restart
for langevin style, damping parameter means small value -> less damping
for nose/hoover style, frequency constant is like an inverse 
  "piston" mass which determines how rapidly the temperature 
  fluctuates in response to a restoring force, large frequency -> 
  small mass -> rapid fluctations
for nose/hoover style, cannot use a end-of-run T of 0.0, must be finite
for langevin and nose/hoover styles, units of frequency/damping constant are
  inverse time, so a value of 0.01 means relax in a timespan on the 
  order of 100 fmsec (real units) or 100 tau (LJ units)

Default = none

thermo flag

print thermodynamic info to screen and log file every this many timesteps
value of 0 means never print

Default = 0

thermo style

determines format of thermodynamic output to screen and log file
Default = 0

timestep

timestep size for MD run (time units)
when rRESPA is enabled, the timestep size is for the innermost (bond) loop

Default = 1.0

true flag

read atom positions (see "read data" command) and dump atom positions
  (see "dump atoms" command) in one of 2 formats
for each dimension, box count of "n" means add that many box lengths
  to get "true" un-remapped position, "n" can be positive, negative, or zero
must be set before data or restart file is read

Default = 0

units

set units to one of two options for all subsequent input parameters

option real = conventional units:
option lj = LJ reduced units:
for LJ units, LAMMPS sets global epsilon,sigma,mass all equal to 1.0
subsequent input numbers in data and command file must be in these units
output numbers to screen and log and dump files will be in these units
this command (if it appears) must be the first command (aside from
  comments) in the input script
must be set before data or restart file is read

Default = real

volume control

styles:
enable volume changes (density changes) during a simulation
specified box boundaries are in distance units
each dimension is controlled separately
dimensions not specified by a "volume control" command can be left
  alone (constant volume or nonperiodic) or controlled by
  a "press control" command
any dimension being varied by volume control must be periodic
the lo/hi values are the desired global simulation box boundaries at
  the end of the simulation run
at each timestep, the box is expanded/contracted uniformly from its initial
  lo/hi values to the specified ending lo/hi values
initial lo/hi values are specified in the data or restart file or 
  inherited from the end of the previous run
at each timestep, all atom coordinates are also scaled to the new box

Default = none