Phase transitions in lattice fluids. I. Short-range interactions

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Abstract
The phase transitions of lattice fluids with molecules defined by first-neighbour exclusion, and interacting at short range, are considered using extended Kikuchi approximations. On the square lattice, a second-neighbour interaction does not produce a disordered low-density transition, but does make the packing transition first order below a critical temperature epsilon beta =-1.05. The resulting phase diagram therefore has only generalized fluid and solid regions. In a Kikuchi double- square calculation with third-neighbour interaction a disordered transition is produced, but this overlaps with the ordered state and again there is no stable intermediate liquid phase. Short-range interactions produce qualitatively the same effect on the triangular and simple cubic lattices. Particularly for the ordered state, the large set of non-linear equations produced by an analytical treatment of extended Kikuchi approximations have limited the range of possible calculations.