Note: Within the molecular mechanics approach, a set of potential functions defining bond stretch, bond angle (both valence and dihedral) distortion energy of a molecule as compared with its nonstrained conformation (that characterized by standard values of bond lengths and angles). A set of transferable empirical force constants is preassigned and the harmonic approximation is usually employed. Some force fields may contain terms for interactions between non-bonded atoms, electrostatic, hydrogen bond and other structural effects as well as account for anharmonicity effects. In vibrational spectroscopy, the inverse problem is solved of determining a set of force constants and other parameters of a chosen potential energy functions which would match with experimentally observed vibrational frequencies of a given series of congeneric molecules.