Lattice gas

The so called `lattice gas' of Lee and Yang was an adaptation of the Ising model, where the spins up and down were replaced by occupied or un-occupied sites (See Ref.s 1 and 2).

References

1. C. N. Yang and T. D. Lee "Statistical Theory of Equations of State and Phase Transitions. I. Theory of Condensation", Physical Review 87 pp. 404 - 409 (1952)
2. T. D. Lee and C. N. Yang, "Statistical Theory of Equations of State and Phase Transitions. II. Lattice Gas and Ising Model", Physical Review 87 pp. 410 - 419 (1952)