Journal of Statistical Physics, Vol. 42, Nos. 5/6, 1986. p. 921-930.
A model classical fluid is constructed by assuming that the direct correlation function c(r-r') is independent of any applied external field. Thermodynamic consistency requires that c(r-r')>/0, and permits explicit representation of the model by a many-body interaction potential. In the canonical ensemble, the model shows a phase transition to an infinite density condensed phase, but in the grand canonical ensemble only an anomalous transition to zero density vapor is found to stably exist.