A whole-path importance-sampling scheme for Feynman path integral calculations of absolute partition functions and free energies

The Journal of Chemical Physics
Steven L Mielke, Donald G Truhlar

Abstract

Using Feynman path integrals, a molecular partition function can be written as a double integral with the inner integral involving all closed paths centered at a given molecular configuration, and the outer integral involving all possible molecular configurations. In previous work employing Monte Carlo methods to evaluate such partition functions, we presented schemes for importance sampling and stratification in the molecular configurations that constitute the path centroids, but we relied on free-particle paths for sampling the path integrals. At low temperatures, the path sampling is expensive because the paths can travel far from the centroid configuration. We now present a scheme for importance sampling of whole Feynman paths based on harmonic information from an instantaneous normal mode calculation at the centroid configuration, which we refer to as harmonically guided whole-path importance sampling (WPIS). We obtain paths conforming to our chosen importance function by rejection sampling from a distribution of free-particle paths. Sample calculations on CH4 demonstrate that at a temperature of 200 K, about 99.9% of the free-particle paths can be rejected without integration, and at 300 K, about 98% can be rejected. We a...Continue Reading

References

Feb 1, 1959·Proceedings of the National Academy of Sciences of the United States of America·D W Jepsen, J O Hirschfelder
Aug 28, 2009·The Journal of Chemical Physics·Jian Liu, William H Miller

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