  Molecular dynamics simulations of water, liquid beryllium fluoride and silica
melt are used to study the accuracy with which the entropy of ionic and
molecular liquids can be estimated from atom-atom radial distribution function
data. All three systems are known to display similar liquid-state thermodynamic
and kinetic anomalies due to a region of anomalous excess entropy behaviour
where entropy rises on isothermal compression. The pair correlation entropy is
demonstrated to be sufficiently accurate that the density-temperature regime of
anomalous behaviour as well as the strength of the entropy anomaly can be
predicted reliably for both ionic melts as well as different rigid-body pair
potentials for water. Errors in the total thermodynamic entropy for ionic melts
due to the pair correlation approximation are of the order of 10% or less for
most state points but can be significantly larger in the anomalous regime at
very low temperatures. In the case of water, as expected given the rigid-body
constraints for a molecular liquids, the pair correlation approximation causes
significantly larger errors, between 20 and 30%, for most state points.
Comparison of the excess entropy, Se, of ionic melts with the pair correlation
entropy, S2, shows that the temperature dependence of Se is well described by T
??2=5 scaling across both the normal and anomalous regimes, unlike in the case
of S2. As a function of density, the Se(rho) curves shows only a single maximum
while the S2(rho) curves show both a maximum and a minimum. These differences
in the behaviour of S2 and Se are due to the fact that the residual
multiparticle entropy, delta(S) = Se - S2, shows a strong negative correlation
with tetrahedral order in the anomalous regime.
