Forward–backward semiclassical dynamics for quantum fluids using pair propagators: Application to liquid para-hydrogen

Forward–backward semiclassical dynamics (FBSD) methods are emerging as a practical way of simulating dynamical processes in large quantum systems. In this paper we develop a pair-product approximation to the coherent state density. This form is accurate at low temperatures, enhancing significantly the convergence of Monte Carlo methods and thus allowing the simulation of quantum fluids. The scheme is applied to the calculation of velocity autocorrelation function of liquid para-hydrogen at several thermodynamic state points (between $T=14 K$ and $T=25 K).$ The results of the forward–backward semiclassical method with the pair-product approximation to the coherent state density exhibit good agreement with experimental measurements and other theoretical calculations. These calculations demonstrate that the FBSD method, in conjunction with an accurate representation of the coherent state density, allows an accurate description of dynamical processes in condensed phase systems at low temperatures where quantum mechanical effects play a significant role.