The most significant problem is the lack of Galilean invariance. This appears in the equations of motion as the non-unity density dependent term . The correct Navier-Stokes equation can be recovered using a scaling technique, see section 3.6, for a single fluid simulation, however for a multi-fluid simulation the scaling procedure will introduce a g factor into the diffusion equation [37, 19]. Thus two fluids can not be correctly modelled in a Galilean invariant system. Some attempts have been made to devise collision rules which achieve = 1 [19, 18] however g remains a function of the density and so the model is only Galilean invariant at one density and can not be used if there is any density variation involved in the simulation. In a single species model the scaling techniques can be used to overcome the lack of Galilean invariance but even here the density must be constant because the viscosity and the pressure are both functions of the density. This severely limits the application of the lattice gas model.