  The behavior of the diameter of the coexistence curve in terms of the entropy
and the corresponding diameter are investigated. It is shown that the diameter
of the coexistence curve in term of the entropy is sensitive to the change in
the character of the rotational motion of the molecule in liquid phase which is
governed by the short range correlations. The model of the compressible
effective volume is proposed to describe the phase coexistence both in terms of
the density and the entropy.
