Ab initionuclear momentum distributions in lithium hydride: Assessing nonadiabatic effects

Abstract

Theoretical nuclear momentum distributions for solid lithium hydride and lithium deuteride are presented. Electronic-structure calculations were performed within the framework of plane-wave density functional theory, followed by the computation of phonon-dispersion relations and vibrational densities of states. The generalized-gradient-approximation functional of Perdew, Burke, and Ernzerhof was used in these first-principles calculations. Our computational results are compared with existing neutron Compton scattering and inelastic neutron scattering experiments on solid LiH. We find an excellent agreement between theory and experiment within the harmonic Born-Oppenheimer approximation. On the basis of the above, we estimate an upper conservative bound of $~$2 to 3% for the effects of nonadiabatic dynamics on the second moment and Laplacian of the atomic momentum distributions in this benchmark system. We close by discussing the implications of this study on future theoretical studies of atomic momentum distributions from isolated molecules and extended condensed-matter systems.