Hello, I try to simulate a fluid by LatticeBoltzmann method. The method works great if there is no external forces are added. But when trying to add an external force like gravity for example. The whole system after few steps are gradually disappear!! I add the force to the overall velocity of some lattice site. Then computer the new equilibrium values for the distribution function!!, like [Buick and Greated, 2000] suggested. The all distribution function values are tend to zero and then tend to negative values!! The whole system Mass, Momentum and Energy must not be lost!! Thanks, Mustafa ELBanna