COMPARATIVE-STUDY OF STRUCTURE DETERMINATION SCHEMES - APPLICATION TOLIQUID ALKALI-METALS

The ionic structure of liquid alkali metals at their melting point is investigated with molecular dynamics, an optimized random phase approx imation (ORPA), and a soft-core mean-spherical approximation (SMSA) as representative of, respectively, simulation, perturbation, and integr al methods. The valence electron screening densities and the interioni c pair potentials are derived from Shaw's optimized model of potential and are free of adjustable parameter. We take advantage of the centra l place of molecular dynamics to analyze the results and point out tha t the interionic potentials issued from Shaw's potential provide a cor rect description of the interactions in the whole set of alkali metals . By comparing simulation results to ORPA and SMSA predictions, it app ears that the use of the last two methods has to be restricted to flui ds with packing fractions of less than about 0.55. The results of the analytical methods are in rather good agreement with simulation and ex perimental data, except with lithium that turns out to have a remarkab ly high packing fraction. Taking advantage of the calculation of the s tructure and the screening charge, the electron-ion pair distribution functions are investigated and features characteristic of alkali metal s are observed.