where P 0 ( N ) ( ξ ξ 1 , ξ ξ 2 , … … , ξ ξ N ) is the probability to find the reference system in the state specified by the variables ( ξ ξ 1 , ξ ξ 2 , … … , ξ ξ N ) . This probability is given by the normalized Boltzmann factor