Consider a system of N particles in a volume V (for an average number density ρ ρ = N / V ) and at a temperature T (let us also define β β = 1 k T ; k is Boltzmann’s constant). The particle coordinates are r i , with i = 1 , … … , N . The potential energy due to the interaction between particles is U N ( r 1 … … , r N ) and we do not consider the case of an externally applied field.