Title: THE STUDY OF QUASISPIN SYSTEMS IN REFERENCE SYSTEM APPROACH
Author(s): I.R.Yukhnovskii, R.R.Levitskii, S.I.Sorokov

For a theoretical description of pseudospin systems with essential short-and long-range interactions a method is proposed. The method is based on calculating of the free energy functional (FEF), where the short-range interactions arc taken as a reference system (RS). The expansions of FEF and the functionals of the temperature cumulant Green function (CGF) in terms of the long-range interaction are investigated. In order to obtain these expansions different types of diagrams are analyzed and summed. For quantum pseudospin systems with arbitrary pseudospin basis we carry out the total summation of block-reducible diagrams in FEF and of non-compact diagrams in the temperature CGF functionals. The scheme of obtaining of so called consistent approximation for thermodynamical and dynamical properties of the system under consideration is discussed in details. Basing on numerical calculation for threedimensional Ising model with unary type reference system it is shown, that for both types of interaction the behaviour of order parameter is described unsatisfactory in one-loops approximation and is acceptable (besides narrow vicinity of phase transition temperature) with taking into account the suming of two-tails diagrams.