A Direct Evaluation of the Partition Function and Thermodynamic Data for Water at High Temperatures

The rovibrational partition function of the water molecule is calculated using a classical statistical mechanics approach and a hybrid method recently proposed by Prudente et al. [J. Phys. Chem. A 2001, 105, 5272], which corrects the classical results. The phase-space integrals are solved using a Monte Carlo technique. For temperatures between 500 and 6000 K, the results are compared with previous approximate and exact quantum calculations. Estimates of some thermodynamic quantities for gas-phase water as a function of temperature are also reported and compared with previous results. The calculated partition function, Gibbs enthalpy, Helmholtz function, entropy, and specific heat at constant pressure indicate that the hybrid scheme can provide accurate thermodynamic data for polyatomic molecules at high temperatures.