Density fluctuations in molten salts. II. Molecular dynamics study of liquid RbBr

Liquid ${\mathrm{Rb}}^{+}$ ${\mathrm{Br}}^{-}$ has been investigated by molecular dynamics for comparison with neutron inelastic scattering results. The properties of ${\mathrm{Rb}}^{+}$ and ${\mathrm{Br}}^{-}$ on the one hand and those of the nuclei of Rb and Br on the other offer rather special features for such a parallel investigation. The results reported here use the Born-Mayer type of interaction between nonpolarizable ions. Based on a molecular dynamics study of a rather small, 122-ion system, our conclusion is that such a force model is probably inadequate for a satisfactory explanation of the neutron scattering data; we also conclude that allowing for ion polarization will impove the situation. Thus detailed studies of the equation of state have to be complemented with dynamical studies of molten salts before one can draw conclusions about the dependability of the Hamiltonian used for the system. Our results for $g_{++}$ and $g_{--}$ show a shoulder to the right of the first maximum. This feature has also been observed in neutron experiments on NaCl and in Monte Carlo calculations on "hard sphere" molten salts. As has already been reported, the calculated and observed static structure factors are in fair agreement. However, a detailed comparison shows more marked features in the calculated results than have been found experimentally. From the density fluctuations a velocity of sound has been deduced which is considerably larger than the only available experimental value. A comparison of the frequency characteristics of the density fluctuations at various wavelengths with neutron scattering data shows that at certain wavelengths there are large discrepancies which are, at least in part, related to the inadequacy of the rigid-ion model Hamiltonian used for these calculations.