Physical Review
Quantum Statistics of Nonideal Systems
Frank H. Stillinger, John G. Kirkwood
Abstract
A new cluster development for the logarithm of the grand partition function of a system of interacting particles is derived. The leading term in this expansion is the pressure exerted by an ideal Bose or Fermi gas at the same temperature and absolute activity $Z$ as the actual system. Succeeding terms involve quantum cluster integrals which themselves depend upon $Z$, unlike their classical analogs. The definition of these cluster integrals follows in a natural fashion using techniques illustrated by construction (in closed form) of the successive $Z$ derivatives of the Bose and Fermi ideal gas grand partition functions. It is not possible (except in the classical limit) to eliminate $Z$ explicitly between the pressure and density series, so that the equation of state must remain in parametric form.