Journal of Statistical Physics, Vol. 62, Nos. 1/2, 1991. p. 35-43.
Exact results are obtained for a lattice gas on the 3-12 lattice with two- and three-site interactions. By using a decoration transformation, we map the lattice gas into one on the honeycomb lattice with pure two-site interactions. This procedure permits us to draw exact results for the original 3-12 lattice gas. In particular, we obtain its exact two-phase boundary, and confirm the fact that an experimentally observed anomaly in the critical behavior of the coexistence-curve diameter is present if, and only if, the three-site interactions are present.