is the potential energy of a non-fixed particle at a distance r 2 of particle 1; Ω 1 and Ω 2 are all the possible orientations the molecules can have in the lattice; Λ, V , N , and β respectively are the de Broglie wavelength, the system's volume, its number of particles, and the reciprocal of k B T , the product of the Boltzmann constant and the absolute temperature.