Approximate quantum dynamics methods for time correlation functions
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The dynamic structure factor of liquid para-hydrogen and ortho-deuterium in corresponding thermodynamic states, (T = 20.0 K, n = 21.24 nm⁻³) and (T = 23.0 K, n = 24.61 nm⁻³) respectively, has been computed by both the Feynman-Kleinert linearized path-integral (FK-LPI) and Ring-Polymer Molecular Dynamics (RPMD) methods and compared with Inelastic X-ray Scattering spectra. The combined use of computational and experimental methods enables a reduction in experimental uncertainties for the determination of the true sample spectrum. Furthermore, the refined experimental spectrum of para-hydrogen and ortho-deuterium is consistently reproduced by both FK-LPI and RPMD at momentum transfers lower than 12.8nm⁻¹. At larger momentum transfers the F K - LP I results agree with experiment much better for ortho-deuterium than for para-hydrogen. More specifically we found that for k ~ 20.0 nm⁻¹ para-hydrogen provides a test case for improved approximations to quantum dynamics. We meet this demand for an improved approximate quantum dynamics method by developing two classes of quasi-classical dynamics that are shown to conserve the initial quantum ensemble when used in conjunction with the Feynman-Kleinert approximation of the density operator. As shown, both classes of dynamics are able to recover the exact classical and high temperature limits of the quantum time correlation function, while a subset is able to recover the exact harmonic limit. A comparison of the approximate quantum time correlation functions obtained from both classes of dynamics are made with the exact results for the challenging model problems of the quartic and double-well potentials. It is found that this new Feynman-Kleinert Quasi-Classical Wigner (FK-QCW) method provides a great improvement over the Feynman-Kleinert implementation of the classical Wigner approximation, also known as FK-LPI, in which purely classical dynamics are used. Furthermore, it is shown that the first class of dynamics reduces to Centroid Molecular Dynamics (CMD) when used within the framework of the classical Wigner approximation for the Kubo transformed time correlation function. Finally, we apply the Feynman-Kleinert Quasi-Classical Wigner (FK- QCW) method to the same liquid para-hydrogen and ortho-deuterium system, previously studied using FK-LPI and RPMD. When applied to this challenging system, it is shown that this new FK-QCW method consistently reproduces the experimental dynamic structure factor for all momentum transfers considered. This shows that FK-QCW provides a great improvement over FK-LPI for not only model problems, but also realistic systems. Furthermore, for small momentum transfers, where RPMD is applicable, it is shown that FK-QCW provides nearly the same results as RPMD, thus suggesting that FK-QCW provides a potentially more appealing algorithm than RPMD since one is not limited to correlation functions involving linear operators. This then suggests that the FK-QCW method is a top contender in the realm of approximate quantum dynamics methods which allow for the practical evaluation of time correlation functions.