Approximation of the n-Vicinity Method

The smallness parameter determining the possibility of application of the n-vicinity method for calculating the free energy of a spin system has been found. The types of spin systems for which approximation of the n-vicinity method is valid have been determined. It is shown that this method is adequate for studying the properties of spin systems, in which the effective number of nearest neighbors is more than $$\frac{{16}}{3}$$ .