The influence of isotopomers on calculated thermodynamics quantities

The effects of using the naturally occurring mixture of isotopomers for calculating thermo-dynamic contributions, rather than using only the compound with the most common isotopes, is examined. The effects are small, even for challenging systems like polyhalogenated compounds. For the most common isotopes the approximation of using only a single isotopomer introduces errors approximately two orders of magnitude less than the neglect of anharmonicity in vibrational frequencies for typical systems.