where m is the mass of the particle and Γ Γ = 6 π π a ρ ρ is the friction coefficient of a diffusing particle, ρ ρ the viscosity. Here Ξ Ξ is the δ δ -correlated Gaussian white noise. The force can derived from a potential well U so that F ( x , t ) = − − U ′ ( x ) and in that case, the equation takes the form