Orbital perspective of the nature of chemical bonds and potential energy surfaces: 55 years after Wahl’s molecular orbital representation of homopolar diatomic molecules

Abstract

To elucidate the molecular electronic energy variation and to simultaneously track the topography of the potential energy surface (PES) such as the potential basin, transition states, and potential barriers in the study of chemical bond formation and relevant chemical reactions, we devise ENOX as an extension of the ENO (Energy Natural Orbital) so that the total sum of the ENOX orbital energies exactly matches the PES energy, that is the sum of the electronic energy and nuclear-nuclear repulsion energy. ENOX gives a one-electron (orbital) energy representation, enabling to track the potential energy surface with the ENOX energies alone. ENOX is applicable to the highly accuracy wavefunctions in ground and excited states, electronic states delocalized across multiple potential energy surfaces (breaking the Born–Oppenheimer approximation), time-dependent wavepacket states, and so on. This paper anatomizes the details of chemical bond formation in the ground state homopolar diatomic molecules in the second row (Li2 to F2) and thereby update the monumental molecular orbital studies by Wahl after 55 years with the ENOX.