Unrestricted primitive model

The unrestricted form of the primitive model (UPM) becomes important for more complex fluid systems. Stell argued that symmetry breaking in the UPM may play an important role in determining critical behavior [17]. In spite of this potential utility, the UPM is rarely explored. In MC simulations of the cluster structure in the UPM, Camp and Patey [259] compared results for asymmetrical charges Xq = z+/z = 1,2,4 at the diameter ratio Xa = vapor phase contains, above all, neutral clusters such as trimers for Xq — 2 and tetrahedral pentamers for Xq = 4, as well as higher clusters. At Xq = 4 asymmetry effects not covered by simple theories seem to play a role. 

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