Nonadiabatic molecular dynamics

Molecular dynamics simulation of mixed quantum-classical systems, in situations where the quantal degrees of freedom undergo transitions among states, poses a number of challenging problems. Among the difficulties are bifurcation of trajectories that evolve into different quantum states and proper treatment of quantum coherence. In this article we outline the problems and contrast the ways in which they are addressed by current methods for nonadiabatic molecular dynamics. In the course of this comparison we present a new result, the relationship between the velocity adjustment in the “surface hopping” method and the “Pechukas force,” as well as some new reflections on an old result, oscillatory yields in ion-surface scattering.