Calculation of the Anharmonicity in the Vibrational Frequencies of Alkane Molecules by the CFF Functions

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Abstract
The CFF method is extended to include all the first five terms of the Taylor expansion of the molecular potential functions. A scheme for anharmonicity calculations in any molecule, in every equilibrium conformation is suggested. The actual anharmonicity calculation for ethane, n‐butane, and cyclohexane is described. It is shown that a mixed function of bond stretching and bond angle bending, with Morse type dependence on the bond length is suitable for such calculations. The advantage of using potential functions instead of a force constant field, which is a central idea of the CFF philosophy, is strongly supported by the calculations.