First principles study of the high temperature partition function and heat capacity of the OH− anion

Abstract

First principles study of the partition function and heat capacity of OH− anion is conducted at high temperatures (2000 K–10,000 K). Firstly, the quantum chemical ab initio methods (RASSCF with CASPT2 correction) were used to calculate potential energy curves of the ground and excited electronic states; the energy points were fitted to analytical representation of the curves. Secondly, the statistical thermodynamics calculations with the classical method with the quantum Wigner-Kirkwood correction were performed to obtain partition function and heat capacity.Thermochemical tables and data sets usually give quantities up to 6000 K (sometimes even more) and often are not reliable at the highest temperatures, it is shown that in particular electronic excited states could be missing. Partition function is compared with the only available Barklem and Collet data set. Discrepancies between heat capacity data (given in NIST-JANAF and Burcat databases) are pointed out - the inclusion of excited electronic states is crucial.