Abstract
Linear screening theory for the interionic interaction in simple liquid metals is extended by including pair and three-ion interactions due to the third-order perturbation term in the electron-ion interaction. Additional pair potentials, phi 2(3)(r), of both sodium and potassium are large and attractive in the short range. The position of the sharply rising part of the pair potential moves to a smaller distance by several percent with the inclusion of phi 2(3)(r). The role of phi 2(3)(r) is essential if the theory of Anderson Weeks and Chandler is to give a good description of liquid structures. On the other hand, second-order calculations are rather successful when the Gibbs-Bogoliubov inequality method is employed.

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