Now consider the equilibrium condition. First, there is no net flow, i.e. J d r i f t + J d i f f u s i o n = 0 . Second, for non-interacting point particles, the equilibrium density ρ ρ is solely a function of the local potential energy U , i.e. if two locations have the same U then they will also have the same ρ ρ (e.g. see Maxwell-Boltzmann statistics as discussed below.) That means, applying the chain rule, ∇ ∇ ρ ρ = d ρ ρ d U ∇ ∇ U .