Influence of the Reaction Field On the Nature of the Phase-transition In the Potts-model

It is shown how Onsager's reaction field may be incorporated in mean-field-like calculations for the q-state Potts model. Its influence on the nature of the phase transition is considered both in a one-site and a two-site cluster approximation. Considerable improvement is obtained for qc(d), the value which separates first- and second-order transitions. The way in which qc(d) is modified when a second-neighbour interaction is added to the ferromagnetic nearest-neighbour one is also studied