Equation (3.27) is not fully isotropic, due
to the tensor 
. Clearly the isotropy of this tensor will depend on the
velocity vectors 
.
Frisch et al. [8] show that for the FHP models the
velocity vectors are such that equation (3.27) is isotropic. This
means that the underlying rotation group of the hexagonal lattice is
sufficiently symmetric to allow
us to model an isotropic fluid. Hexagonal grids
are generally used in lattice gas simulations because they are the
simplest lattice for which the fluid is isotropic.