A new model for the ground state potential energy curves of diatomic molecules

A new model for the ground state potential energy curves of heteronuclear diatomic molecules is presented. In this model, applicable equally to covalent and ionic bonds, the total potential energy is decomposed into a covalent and an ionic component whose respective contributions are determined by a quantity [function], called bond polarity, which is an accurate measure of the ionic character of a bond. The ionic part of the energy is modeled by the Rittner potential, derived using the implicit perturbation theory formalism, while the covalent part is represented by an empirical potential energy function whose parameters are related to atomic constants obtained from the equilibrium spectroscopic properties of the homonuclear diatomic molecules corresponding to the atoms forming the heteronuclear molecule. The new model is applied in detail to the alkali halides. Employing the Hulburt-Hirschfelder potential energy function to represent the covalent component of the energy, significant improvement in the predicted values of the spectroscopic constants of these molecules is achieved over the earlier models which all consider the alkali halides 100% ionic molecules. An expression of the model in terms of atomic and ionic constants only is obtained by decomposing the repulsive part of the ionic component of the energy into singleion contributions according to an arithmetic mean rule related to ionic radii. The model is also applied to some essentially covalent heteronuclear molecules containing two alkali or halogen atoms. Satisfactory agreement with experiment is found and in many instances the results are superior to ab initio calculation results.