Refinement of the experimental dynamic structure factor for liquid para-hydrogen and ortho-deuterium using semi-classical quantum simulation

Abstract

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 enabled us to reduce experimental uncertainties in 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 results at momentum transfers lower than 12.8 nm(-1). At larger momentum transfers the FK-LPI 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.

Application of a new ensemble conserving quantum dynamics simulation algorithm to liquid para-hydrogen and ortho-deuterium

“…We then provide a comparison between the present calculations and the experimental determinations, as well as with the ones obtained by RPMD and FK-LPI as previously published in Ref. 7. Specifically, it was found in Ref.…”

“…III, we begin by discussing the computational details of our simulations, followed by a comparison of the FK-QCW method with the experimental dynamic structure factor and that obtained by RPMD and FK-LPI previously published in Ref. 7. The conclusions are presented in Sec.…”

“…Low-temperature liquid para-hydrogen and orthodeuterium have become benchmarks in the development of approximate quantum dynamics methods [1][2][3][4][5][6][7] that allow for the practical evaluation of a general quantum time correlation function of the form  (0)B(t) = 1 Z Tr ( e −βĤ e iĤ t / B e −iĤ t / ) ,…”

On the non-Gaussian corrections in the self dynamics of semi-quantum fluids

“…On the computational side, centroid molecular dynamics [3] and ring polymer molecular dynamics [4] are surely among the simulation techniques producing the best results for semi-quantum liquids, but their scope is limited to evaluate the time-correlation functions of operators linear in position,rðtÞ, or momentum,pðtÞ, only. The Feynman-Kleinert linearized path-integral does not seem to suffer these limitation [5], but its capability to precisely reproduce the dynamic structure factors of liquid hydrogen and deuterium is still matter of discussion [6]. Among the various promising approaches to the collective and the self dynamics of semi-quantum liquids, two kinds of methods are surely worth mentioning: those based on the so-called quantum generalized Langevin equation [7,8], which extend the mode coupling theory beyond classical systems, and those exploiting the analytic continuation of the imaginary time correlation functions, which make use of advanced maximum entropy methods [8,9].…”

Gas phase infrared spectra from quasi-classical Kubo time correlation functions

“…The available methods differ in the procedure used to simplify the dynamics or in the way in which the thermal density is represented and, importantly, sampled. The classical approximation of the dynamics is valid for short times [5,6], and quasiclassical methods have been successfully applied to study processes in which the time correlation function decays to zero fast enough to make the inadequacies of the evolution irrelevant [7][8][9][10]. In particular, we recently showed that the quasi classical phase integration method [11] (PIM) can be used to obtain infrared spectra of small molecules in the gas phase with good accuracy [12].…”