Abstract

Molecular dynamics (MD) simulations of complex systems can now be performed routinely and often with great success. Unfortunately, Newton’s law of motion will not adequately describe the dynamics of molecules, due to quantum effects, which become important for low temperature and light nuclei. Within the last decade, new methods have been developed for including quantum effects into MD. Besides the successful Ring Polymer Molecular Dynamics scheme (RPMD), Wigner’s distribution function in the canonical ensemble has often been a popular choice for generating quantum initial conditions for classical MD. These schemes work by sampling phase-space points quantum mechanically and afterwards propagate them forward in time by classical MD. Such approaches have suffered from the so-called “ensemble conservation problem”, meaning that the propagated phase-space points no longer represent the canonical ensemble. Hence, these methods are dynamically inconsistent. In this talk, I will present an approach, still based on the Wigner function, which conserves the ensemble. The method is used for obtaining the dynamic structure factor of liquid para-hydrogen and ortho-deuterium at low temperatures. We compare the results with experiment.