The gradient expansions of the kinetic energy and the mean momentum for light diatomic molecules

Abstract

For eleven neutral diatomic systems and one positive ion we have numerically evaluated the three terms in the gradient expansion of the molecular kinetic energy T=T₀+T₂+T₄. Hartree–Fock–Roothaan molecular densities were used throughout. For the molecules studied T₂/T₀ ∼0.1, T₄/T₂ ∼0.2. The contributions to these integrals from different regions of space were examined. We have also estimated the values of the two terms in the gradient expansion of the mean momentum P=P₀+P₂ and find P₂/P₀ ∼0.5. Lastly we make contact with earlier work of Mucci and March. D/N² is found to correlate grossly with T₂, where D is the dissociation energy and N the total number of electrons in the molecule.