Forward–backward semiclassical dynamics for condensed phase time correlation functions

The forward–backward semiclassical dynamics (FBSD) scheme for obtaining time correlation functions shows much promise as a method for including quantum mechanical effects into the calculation of dynamical properties of condensed phase systems. By combining this scheme with a discretized path integral representation of the Boltzmann operator one is able to calculate correlation functions at finite temperature. In this work we develop constant temperature molecular dynamics techniques for sampling the phase space and path integral variables. The resulting methodology is applied to the calculation of the velocity autocorrelation function of liquid argon. At the chosen state point the FBSD results are in good agreement with classical trajectory predictions, but the existence of a non-negligible imaginary part of the correlation function illustrates the importance of proper density quantization even under nearly classical conditions.