eFF is a molecular dynamics model that includes electrons . It can simulate highly excited systems where the Born-Oppenheimer approximation may break down over long times, and where excitations may be distributed in a spatially heterogeneous nonequilibrium fashion over tens of thousands of electrons. Applications include modeling shock compressed dense matter, semiconductor etching by plasmas, and radiation damage of materials (Figure 1).
Only three universal parameters are used, for all elements. Currently, elements with Z=1-6 are well described. Work is ongoing to extend the scope of the force field, with preliminary results for Z=1-10 detailed in a separate section (eFF2).
In ground state systems -- and excited state systems between curve crossings -- the Born-Oppenheimer approximation applies, and the electronic wavefunction is set by the instantaneous position of the nuclei. In conventional force fields, the total energy is parameterized as a function of the nuclear coordinates alone. Since a wavefunction does not need to be explicitly computed, a significant time savings over quantum mechanics methods is possible, and the dynamics of large systems can be simulated in a practical way.
However this approach fails to describe highly excited systems, where there may be many curve crossings and a high density of states, causing the Born-Oppenheimer approximation to break down over long time intervals (Figure 2).
In eFF, a very simple time-dependent wavefunction containing both nuclear and electron parameters of motion is used. Substituting the wavefunction into the time-dependent Schrodinger equation generates semiclassical equations of motion which specify how nuclear positions and electron positions and sizes change over time.
Since electrons and nuclei in eFF can move independently of each other, the method can simulate the chemistry of large scale excited systems undergoing nonadiabatic as well as adiabatic dynamics. The energy function is simple enough that the speediness of conventional force field/MD schemes is preserved, which allows practical computations on large systems (up to millions of electrons, Figure 3).
Heller pioneered the use of semiclassical wave packet dynamics of nuclei to solve spectroscopic problems in a time-dependent framework; eFF uses equations of motion similar to his to propagate nuclear and electron coordinates .
eFF falls into a category of methods called ``fermionic molecular dynamics'' , where the interaction energy of fermions is computed using effective potentials, with particular emphasis placed on accounting for the effects of Pauli exclusion and Heisenberg uncertainty.
These methods have been used to study the scattering and combination of nucleons in nuclear reactions (Dorso and Randrup [4,5], Boal and Glosli [6,7], Maruyama et al , Wilets et al ); the dynamics of hydrogen plasmas and liquid lithium (Hansen and McDonald , Klakow and Knaup [11,12]); atomic shell structures (Kirschbaum and Wilets , Cohen ); and proton passage through beryllium (Beck and Wilets ).
eFF represents an advance over these methods in that it is accurate enough to reproduce a wide variety of ground state structures containing covalent, ionic, multicenter, and/or metallic bonding; and further that its excited state dynamics for warm dense hydrogen  and core-ionized hydrocarbons show good agreement with experiment and higher level theory.
eFF is also related to the ab initio floating spherical Gaussian orbitals (FSGO) methods developed by Frost in 1964, who showed that it was possible using a minimal basis set of floating Gaussians to obtain reasonable geometries of hydrocarbons and atom hydrides . Overall, we find that eFF produces geometries as good or better than FSGO, with much reduced computational cost ( versus , if no cutoffs are used).