Canonical Dynamics Simulations of Single-Chain Polyethylene
Cagin-T, Goddard-WA, Ary, ML
Computational Polymer Science, 1, 241-248 (1991).
Conventional microcanonical molecular dynamics (Newtonian dynamics)
generates trajectorie confined to a constant energy surface in a
6N-dimensional phase space ( N = number of atoms ). In contrast,
canonical dynamics (Nose) utilizes a modified lagrangian which can
be shown to yield a canonical distribution of points in phase space.
The total energy fluctuates in canoncial dynamics, leading to a rapid
equilibration of polymer conformations. We illustrate this feature of
canonical dynamics by applying it to study the equilibrium distribution
of conformational states of medium length (122 to 302 atoms), single
chain ethylene oligomers. For T=300 K, we find that canonical molecular
dynamics reaches equilibrium thermal distributions in about 10 ps
whereas microcanonical dynamics has not produced an equilibrium
distribution in about 1ns simulation. The equilibrium distributions
calculated by time-averaging canonical molecular dynamics over
10-30 ps agree with each other and with the results obtained from
static molecular mechanics calcualtions (using the same classical
interaction potential for polyethylene).
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