Accurate partition function and thermodynamic data for water

Abstract

The partition function, Q, of H216O is calculated by explicit summation of about 10 500 experimental vibration-rotation energy levels and very high accuracy estimates are obtained for the specific heat capacity (Cp), the Gibbs enthalpy function (gef), the Helmholtz function (hcf) and the entropy (S) of gas phase water as a function of temperature. For temperatures above 600 K it is necessary to augment the sum with theoretical estimates of the energy levels. These are obtained from high accuracy variational calculations which are extended to dissociation using a model for rotational levels based on a Padé approximant. Estimates for the partition function and other thermodynamic quantities are obtained for temperatures up to 6000 K and temperature dependent error bars presented. All estimates are highly accurate with the exception of Cp for T>5000 K, for which further work is required.