Molecular dynamics analysis of incoherent neutron scattering from light water via the Van Hove space-time self-correlation function with a new quantum correction

In this paper, we propose a general method for evaluating the neutron incoherent scattering cross-section of light water by molecular dynamics (MD) analysis of the Van Hove space-time self-correlation function (STSCF) with a newly developed quantum correction named as Gaussian approximation-assisted quantum correction (GAAQC). The self-intermediate scattering function (SISF) by GAAQC satisfies the detailed balance condition and sum rules up to second order adequately, and approaches for long times the one obtained directly from MD trajectory data. These features are desirable for the evaluation of the neutron scattering cross-section over a wide range of energy and momentum transfer. From the analysis of quasi-elastic scattering, the self-scattering law by GAAQC shows the jump-diffusive behavior of the molecular translational motion, which is not reproduced by Gaussian approximation (GA). As compared with GA, double differential and total cross-sections by GAAQC show better agreement with experimental data, particularly below the cold neutron region where the non-Gaussian property of the SISF becomes apparent. Thus, the present method, namely, the direct analysis of the STSCF with GAAQC will serve for improving the evaluation of the neutron scattering cross-section for light water.