Calculation of thermodynamic functions of the three-dimensional Ising ferromagnet above and below critical temperature is performed in the approximation of sixfold basis distribution (ρ 6 model). Comparison with the results for the ρ 4 model indicates that dependence of the thermodynamic functions on the renormalization group parameter s becomes weaker. The optimal interval of the renormalization group parameter values is determined.

The critical behaviour of the three-dimensional Ising-like system on a simple cubic lattice with an exponentially decreasing interaction potential is investigated within the collective variables method. The solution of the equation for the phase transition temperature is obtained by taking into account the dependence of its coefficients on the temperature. The dependence of the critical temperature on the interaction potential range is studied. 05.50.+q, 64.60.Fr, 75.10.Hk This paper gives rise into the theory of second order phase transitions in the approach developed by I.R.Yukhnovskii [1]. In a number of recent papers [2][3][4][5], the explicit expressions for basic thermodynamic functions of the three-dimensional Ising-like model have been obtained. In particular, the dependence of these functions on τ (τ = (T − T c )/T c , T c is the phase transition temperature), as well as on microscopic parameters of the model, was studied. On the other hand, the dependence of T c itself on microscopic parameters has not been studied deeply enough so far. Such a study is our aim in this research. Key words: Ising-like systems, critical behaviour, phase transitions, collective variables, phase transition temperature PACS:We investigate a system of Ising spins located at the sites of a simple cubic lattice L:The formal Hamiltonian of the system isHere h is an external field, r ij is the distance between the particles at the sites i and j, σ i is an operator of the z-component of a spin in the i-th site. The interaction