Molecular dynamics simulation of hydrogen absorbing palladium: Turnover ofhydrogen absorption heat into palladium

Molecular dynamics simulations were carried out for hydrogen absorption by palladium in various atomic ratios to identify the reason why hydrogen abso rption heat changes from exothermic to endothermic at a high atomic ratio. Classical two-body interactions were used as interatomic potentials for the se simulations in order to simplify the calculation. Pd-Pd and Pd-H interat omic potentials were determined empirically by fitting the calculated physi cal properties of PdHx's with observed values at an atomic ratio, and the H -H potential was given by a screened Coulomb potential. Calculated molar vo lumes and absorption enthalpies were consistent with observed data for a wi de range of atomic ratios. These results show that the simple approach was sufficient to evaluate the absorption heat of PdHx. Analyses of crystal str ucture and the averaged H-H coordination number indicate that the change of the absorption heat from exothermic endothermic was not caused by a struct ure change, but by H-H repulsion.