PAD APPROXIMATION IN A THEORY FOR ISING PAIR-QUARTET INTERACTIONS

The self-consistency of the PAD approximation used recently by Mitran (see ibid., vol.12, p.557, 1979) in conjunction with the generalised non-uniform magnetic field method for Ising pair-quartet interactions, is critically examined. This is done by using the method described by Mitran to generate an equality between two different four-spin correlation functions, one of which was originally used by Mitran to obtain agreement with series results obtained by Griffiths and Wood (1973) to within 0.3%. Application of the PAD to the two correlation functions leads to two values of Tc (for the FCC lattice in the quartet limit) which differ by 40%. It is therefore suggested that the apparent agreement with the series results should be treated with caution. This view is supported by a comparison between the results obtained by Mitran with those given by Griffiths and Wood for a BCC system.