Quantum hydrogen vibrational dynamics in LiH: Neutron-scattering measurements and variational Monte Carlo simulations

Hydrogen single-particle dynamics in solid LiH at T=20K has been studied through the incoherent inelastic neutron-scattering technique. A careful analysis of the scattering data has allowed for the determination of a reliable hydrogen-projected density of phonon states and, from this, of three relevant physical quantities: mean-squared displacement, mean kinetic energy, and Einstein frequency. In order to interpret these experimental findings, a fully quantum microscopic calculation has been carried out using the variational Monte Carlo method. The agreement achieved between neutron-scattering data and Monte Carlo estimates is good. In addition, a purely harmonic calculation has been also performed via the same Monte Carlo code, but anharmonic effects in H dynamics were not found relevant. The possible limitations of the present semiempirical potentials are finally discussed.