Development of a universal Electron Force Field

In many cases, the unavailability of an adequate classical force field is a major barrier for molecular dynamics simulations. In these cases, density functional theory (DFT) provides a universal but expensive option. We propose an intermediate approach based on an ‘electron force field’ (eFF) [Su and Goddard, PRL 99, 185003 (2007)]. Inspired by VSEPR theory, we expand the electron density as a sum of overlapping spherical electron ‘balls’ (e-balls). The e-balls interact through electrostatic forces, Pauli repulsion, and exchange-correlation. The electronic total energy is calculated as a universal function of the e-ball positions and widths. As in DFT, it is independent of the nuclei, which interact only electrostatically. We parameterize this universal, many-body function by fitting to DFT calculations of a large number of molecules and solids at equilibrium, distorted, and reaction geometries.