A remedy for zero�point energy problems in classical trajectories: A combined semiclassical/classical molecular dynamics algorithm

A new method is proposed for dealing with difficulties in molecular dynamics (MD) simulations caused by nonpreservation of zeroâ€�point energies (ZPE) in classical dynamics. Specifically addressed is a difficulty, for molecules held in weakly bound clusters, of energy flow from the initial ZPE of stiff molecular vibrations into soft cluster modes, causing unphysical dissociation or melting of the cluster. The remedy proposed is a classicallike MD algorithm, which treats the stiff modes by semiclassical Gaussian wave packets and the soft modes by classical dynamics, using the timeâ€�dependent selfâ€�consistent field (TDSCF) approach to couple the classical and the semiclassical modes. The resulting algorithm is very similar in form to classical MD, is computationally simple, stable, and appears free of unphysical effects. The method is illustrated by test applications to models of the clusters I₂He and (HBr)₂ in the ground states, which dissociate at the expense of their ZPE classically, but remain stable in the new method.