The enthalpy and heat capacity of liquid water and the ice polymorphs from a random network model

Abstract

The enthalpy and heat capacity of liquid water are calculated from the distributions of static hydrogen bond angles and lengths which characterize the random network model (RNM) previously introduced. As a test of consistency the energies of ices II, III, V, and VI relative to ice I are also computed; the average deviation between experiment and theory is only 0.7 kJ/mol. The enthalpy of liquid water, calculated in the same fashion, agrees with the observed values over the temperature range 0–100 °C. Three calculations of the heat capacity, from the entropy, the enthalpy, and fluctuations in the enthalpy, yield internally consistent values which are in good agreement with experimental data. Our results indicate a large contribution to the heat capacity from the strong coupling between vibrational and configurational contributions to Cp, which coupling is a reflection of the quantized molecular dynamics.