  The study of Mayer's cluster expansion (CE) for the partition function
demonstrates a possible way to resolve the problem of the CE non-physical
behavior at condensed states of fluids. In particular, a general equation of
state is derived for finite closed systems of interacting particles, where the
pressure is expressed directly in terms of the density (or system volume) and
temperature-volume dependent reducible cluster integrals. Although its accuracy
is now greatly affected by the limited character of the existing data on the
reducible cluster integrals and, especially, the absence of any information on
their density dependence, a number of simple approximations indicate the
qualitative adequacy of this equation in various regimes of a fluid: from
gaseous to liquid states (including the transition region).
