eFF is a force field that contains electrons. Nuclei are represented by point charges, and electrons by spherical Gaussian wave packets whose position and size can vary, and which can be tagged with either an up or down spin. The particles interact via an effective potential that is a sum of electron kinetic energies, electrostatic interactions, and Pauli repulsion. This version of eFF gives a reasonable description of systems containing nuclei from Z=1-6.

To compute the ground state, we minimize the energy of the system, optimizing the nuclear and electron positions and the electron sizes simultaneously. To simulate materials at finite temperatures, including nonadiabatic excited electron dynamics, we propagate semiclassical Ehrenfest dynamics, allowing the positions of the nuclei and electrons, as well as the sizes of the electrons, to change with time.

The eFF program has the following features:

*Single point energies and forces*for a given configuration of nuclei and electrons.*Minimization*to obtain ground states. Nuclei and electrons can be optimized together, or just electrons alone.*Dynamics*to propagate excited electron dynamics. Both NVE and NVT dynamics can be performed.- Computation of
*virial functions*such as instantaneous pressure, and for dynamics, temperature. *Constraints*on nuclei and electron positions, or on electron radii, applied during minimization and dynamics.*Restraints*on a variety of nuclear/electron distances and angles applied during minimizations and dynamics.- Setting of
*initial velocities*for electrons and nuclei. - Application of
*time varying electric fields*. - Application of rectangular
*periodic boundary conditions*. eFF uses Ewald summation for electrostatics, and the minimum image convention for Pauli repulsion. - Application of
*pairwise cutoffs*(optional). This enables linear scaling dynamics, making it practical to perform dynamics on nonperiodic and periodic systems containing hundreds of thousands of electrons. - Memory usage is linear with the total number of nuclei and electrons. All memory is dynamically allocated, so that the system size is limited only by the memory installed in the computer.

We have included several supporting programs. The **eFFview** visualizer makes it easy to generate high quality still frames and movies of nuclear and electron dynamics from eFF input and output files. Builder scripts such as **xyz2cfg**, **diamond_cfg**, **beryllium_cfg**, etc. can be run to create a wide variety of molecules and periodic extended structures for simulation with eFF (Chapter 5).