Quasi-particle picture for monatomic gases

A quasi-particle theory for monatomic gases in equilibrium is formulated an d evaluated to yield the exact virial contributions to the thermodynamic st ate functions in lowest order of the density. Van der Waals blocking has ne cessarily to be accounted for in occupation number statistics. The quasi-pa rticle distribution function differs from the Wigner function by a bilinear functional thereof. The progress made so far is promising with respect to a corresponding version of kinetic theory. (C) 1999 Elsevier Science B.V. A ll rights reserved.