Simulations of hydrogen diffusion in Pd were performed for a system consisting of 256 Pd atoms and 8 H atoms at 623K (table 18). Under these conditions, detailed quasi-elastic neutron-scattering data were available. For the interatomic interactions, the embedded-atom method was used which incorporated some essential many-body effects in metals. Based upon the embedded-atom method approach, the wave-vector dependence of the width of the quasi-elastic neutron-scattering peak was investigated in detail. It was found that a single electronically adiabatic potential-energy surface could not reproduce the observed wave-vector dependence. After incorporating the coupling of hydrogen atoms to the low-lying electron-hole pair excitations among the conduction electrons, close agreement with the experimental data was obtained. This was a strong indication that one had to go beyond the Born-Oppenheimer approximation in order to characterize correctly the diffusive motion of hydrogen in metals. To reveal the diffusive behavior in more detail, the residence time distribution and the correlation character in diffusion direction were investigated. It was found that including the non-adiabatic corrections reduced the probability for the H atoms to move over several lattice sites without getting trapped in between. As a result, the motion of the H atoms became more similar to that assumed in the Chudley-Elliott model, which described well the quasi-elastic neutron-scattering data for the wave-vector dependence of the width.

Molecular-Dynamics Simulation of Hydrogen Diffusion in Palladium. Li, Y., Wahnström, G.: Physical Review B, 1992, 46[22], 14528-42