where a and b denote the fluid species and can be either A and B, ϵ ab describes the interaction between fluid a Solid lattice site: occupied by a solid particle Fluid lattice site: occupied by a fluid particle FIG. . An example of a 4×4 cubic lattice system, red and gray spheres are solid and fluid particles, respectively. and b. The fluid occupancy for site i is denoted as n i which can be either 0 (unoccupied) and 1 (occupied), j represents the nearest neighbour of site i (j ̸ = i). The external field at site i, ψ i , can be expressed as