An interpretation of potential interaction constants in terms of low-lying excited states

This paper examines the use of second-order perturbation theory in the interpretation and prediction of the signs of the interaction constants which are appended to the simple valence force potential functions. It is shown, for the large number of molecules considered here, that complete agreement with the observed signs can be obtained by expanding the electronic Hamiltonian in terms of the normal coordinates of motion and by then assuming that the lowest of the excited electronic states makes the most important contribution to the second-order sum in the expression for the energy. A knowledge of the symmetry of the transition density between the ground and the first excited electronic states is thus sufficient to determine which of the vibrations in a molecule is energetically favoured and thus to determine the sign of the interaction constant.