Core polarization and the structure of simple metals

Abstract

Many liquid simple metals (Ga and Sn are prominent examples) have static structure factors $S(k)$ that appear to depart appreciably from the predictions of standard central-force models of liquid structure. The inferred structural anomalies take the form of weak subsidiary maxima on $S(k)$ near the principal diffraction peak, or even asymmetries in the peak itself. They are shown here to be a consequence of corresponding structure introduced into the effective ion-ion interaction by the inclusion of dynamically screened fluctuating dipole interactions between ion cores. The effects are therefore most pronounced in ions that are highly polarizable and in extreme cases actually manifest themselves as nonmonotonic contributions to the pair potential at relatively short range. Less extreme behavior can result merely in a softening of the standard pseudopotential-based pair interaction calculated in the absence of such terms. A calculation of the effective ion-ion potential therefore requires a knowledge of the frequency-dependent ionic and electron gas polarizabilities and the one-electron pseudopotential. Model potentials based on reasonable estimates of all of these are used in conjunction with simulation (Monte Carlo) methods to show that the structural anomalies can indeed be attributed to core-polarization effects.