Inelastic neutron scattering from liquid para-hydrogen by ring polymer molecular dynamics

We investigate the use of the ring polymer molecular dynamics (RPMD) method to calculate the incoherent dynamic structure factor of liquid para-hydrogen at a temperature of 14 K. Two different approximation schemes are considered: the RPMD approximation to the self relaxation function (F) over bar (s)(kappa, t) and the Gaussian approximation to the intermediate scattering function F-s(kappa, t). We find that both approximations give similar results for low momentum transfer, but that their predictions differ appreciably for kappa > 1.5 angstrom(-1). Exact moment constraints are then used to differentiate between the two approximations. These constraints suggest that the Gaussian approximation to F-s(kappa, t) employing the RPMD velocity autocorrelation function is the more accurate of the two approximations for large kappa. We end by comparing the results of this approximation with those of a recent inelastic neutron scattering experiment involving two different kinematic lines in the (kappa, omega) (momentum and energy transfer) plane. (c) 2005 Elsevier B.V. All rights reserved.