Molecular dynamics studies of neptunium dioxide

The molecular dynamics (MD) calculation has been performed for neptunium dioxide (NpO2) in the temperature range from 300 to 2500 K to evaluate the thermophysical properties viz., the lattice parameter, thermal expansion coefficient, compressibility, heat capacity, and thermal conductivity. The Morse-type potential function added to the Busing-Ida type potential was employed for the ionic interactions. The interatomic potential parameters were determined by fitting to the experimental values of the lattice parameters of NpO2. The thermal expansion coefficient and compressibility were calculated from the temperature and pressure dependences of the lattice parameter, evaluated from the fixed pressure MD calculation. The heat capacity at constant volume (CV) was calculated from the variation of the internal energy of the system, evaluated from the fixed volume MD calculation. The lattice dilation contribution to the heat capacity (Cd) was evaluated from the calculated thermal expansion coefficient and compressibility. The thermal conductivity was calculated by using a Green-Kubo relation.