EXTENDED HELLMANN-FEYNMAN THEOREM FOR NONSTATIONARY STATES AND ITS APPLICATION IN QUANTUM-CLASSICAL MOLECULAR-DYNAMICS SIMULATIONS

An extended Hellmann-Feynman theorem for non-stationary states is presented. For systems in which some particles are described quantum-mechanically and others classically, the extended theorem allows one to determine more precisely the forces exerted by quantum particles on classical particles. The forces are used in quantum-classical molecular dynamics (QCMD) simulations, where the extended theorem allows an increase in the integration time-step and a reduction in the energy drift. The QCMD model also accounts for a dynamical vibronic coupling between quantum and classical subsystems, which further improves the quality of the simulation results.