Bipolarons in a KCl melt

A theory of two excess electrons in alkali halide melts is developed using variational estimates of path integrals. As a result of the strong screening, the average field generated by the ions has little influence on the electrons and the problem reduces to a study of a bipolaron type of free energy functional. The behavior of this functional is determined as a function of the thermodynamic and structural characteristics of the melt. Variational bipolaron calculations are made using the approximation of uncorrelated electrons and using Kohn-Sham theory to allow for electron-electron correlations. The results of the calculations using Kohn-Sham theory agree with the data obtained by quantum molecular dynamics and show that a correct choice of trial wave function which allows explicitly for the correlation of two electrons is required to obtain a correct estimate of bipolaron stability.