  In contrast to the gaseous and the solid states, the liquid state does not
have a simple model that could be developed into a quantitative theory. A
central issue in the understanding of liquids is to estimate the energy state
of liquids. Here, on the basis of our recent studies on crystal melting, we
show that the energy sate of liquids may be reasonably approximated by the
energy and volume of a vacancy. Consequently, estimation of the liquid state
energy is significantly simplified comparing with previous methods that
inevitably invoke many-body interactions. Accordingly, a possible equation of
state for liquids is proposed. On this basis, it seems that a simple model for
liquids is in sight.
