Displaced-points path integral method for including quantum effects in theMonte Carlo evaluation of free energies

A semiclassical approximation is proposed for the Monte Carlo quantum mecha nical path integral approach to the calculation of equilibrium properties ( partition functions, free energies) of molecules or generalized transition states. The approximation is based on replacing the sum over paths by a sum over points that are displaced from the classically sampled points. The di splacements are adjusted to ensure the correct results for harmonic potenti als, and thus the method is akin to perturbation theory with a harmonic ref erence. In its simplest form, the approximation takes the form of a classic al mechanical calculation with double sampling for every configuration poin t; thus it can be readily adapted to a variety of applications. Calculation s by several other approximate methods are also presented, and the results with the new method compare very favorably with them. (C) 2001 American Ins titute of Physics.