Effects of anharmonic vibrations on molecular properties

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Abstract
In consequence of recent advances made mainly in ab initio methods, relatively accurate anharmonic force fields are beginning to be available for an increasing number of molecules. This makes the calculation of anharmonic vibrational averaging effects on various molecular properties practical. In the present paper this calculation is discussed in detail, pointing out the simplifications which result when the effects of anharmonicity are specified by the conversion between the equilibrium and average geometries of the molecule. The different contributions to the vibrational averages of the internal coordinates of the molecules HCN, C2H2, CH4, C6H6 and NH3 are reported, considering their sensitivity to the details of the force fields used in the calculation. By combining the results with the experimental structures of these molecules their equilibrium or average geometries are derived.