These are quantum-statistical expressions containing kinetic energies. Note that the one-particle partition function Q 1 contains only a kinetic energy term. In the classical limit ℏ ℏ = 0 the kinetic energy operators commute with the potential operators and the kinetic energies in numerator and denominator cancel mutually. The trace (tr) becomes an integral over the configuration space. It follows that classical virial coefficients depend on the interactions between the particles only and are given as integrals over the particle coordinates.