A new directional model for the electronic frictional forces in molecular dynamics simulations of radiation damage in metals

The damage caused by collision cascades in irradiated materials forms the initial conditions for longer term microstructural evolution. The exchange of energy between electrons and ions during cascades can significantly affect this damage. Models for incorporating this exchange within classical molecular dynamics (MD) simulations exist, but most approximate the ion-electron energy transfer via a damping force, opposed to ionic motion. Although such forces predict the total energy transfer over the duration of cascades, they do not capture the complex dependence of the non-conservative electronic friction force on the speed, direction and atomic environment of individual ions. Here we present a new model for the electronic friction force, derived from quantum-classical Ehrenfest dynamics, which captures this complexity and is suitable for inclusion in existing MD codes at near-zero computational cost. We show that our model reproduces the atomic level detail of the nonconservative electronic force in time-dependent tight-binding simulations of cascades.