text: The general equation can then be written as
  where the "force" term corresponds to the forces exerted on the particles by an external influence (not by the particles themselves), the "diff" term represents the diffusion of particles, and "coll" is the collision term – accounting for the forces acting between particles in collisions. Expressions for each term on the right side are provided below.
  Note that some authors use the particle velocity  instead of momentum ; they are related in the definition of momentum by .