The expressions of the free energy and the pair correlation functions for random binary alloys are obtained by means ofthe method of collective variables. The advantages of a new approach are discussed. The theory is exemplified by numerical calculations of the K-Cs system properties. The calculated phase diagram is in agreement with the results obtained by other authors and with the experimental data. The analysis of the Fourier transform of the binary correlation function shows that the K,Cs, --c alloys are similar to ideal solid solutions at high temperatures ( T 2 300 K). As the temperature decreases, the deviation from the ideal solution increases. The short-range order parameter at the first coordinate sphere is calculated.

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An original method of treating the kinetic and exchange-correlation energies functionals in terms of many particle interactions was developed. It is based on utilizing the local density approximation. The total electron density, extracted from the ab initio .band structure calculations, is expressed as a linear superposition of contributions from the individual pseudoions embedded in the uniform background. The explicit expressions for the pair and triplet potentials are presented. The general form for the part of the pair interatomic interactions caused by the kinetic and. the exchange-correlation effects is obtained. Relationship between the developing approach and the perturbation theory in pseudopotential is analysed. Unlike the perturbation theory the advanced approach allows one to calculate accurately the so-called reducible contributions to the pair potential arising from the n-particle (n > 2) inter-. actions. It corresponds to summing certain series in pseudopotential. Contribution of the electron-nonlocal pseudopotential interactions to the pair interatomic ones is considered within the concept of the fully separable pseudopotentials.

A microscopic approach to considering the influence of atomic local static displacement (Am) on binary alloy thermodynamic propalies has been developed. An quation that permils the calculation of ASD amplitudes from first principles has been derived. The value of the ASD amplitude is shown to depend on the difference in the effective interatomic potentials of the alloy componens. The expression for the alloy free energy, which lakes the ASD into account, is oblaiaed by the collective variables method. The theoretical results are illustrated by numerical calculations, performed for alloys of the K-Rb system. The dependence of the ASD squared. avenged over configurations, on temperahlre and alloy concentration has been investigated, in particular.

Lattice dynamics of binary alloys is investigated by the collective variables method. The relation between a developing technique and the well-known virtual crystal and coherent potential approximations is analysed. The characteristic property of the theory presented consists in taking into account explicitly the short-range order. The influence of pair correlations on the vibrational density of states and the Debye temperature for K-Rb alloys is calculated and discussed. The short-range-order effects on the dynamic properties are shown to be strong near the order-disorder phase transition temperature.