Chemical Bonding from the Perspective of <i>In Situ</i> Orbital Correlation

We propose a novel concept of "<i>in situ</i>" orbital correlation to gain a deep understanding of the nature of the chemical bond. In stark contrast to popular traditional orbital correlations, where the orbital energies are derived from the free and noninteracting states of isolated species, the "<i>in situ</i>" orbital correlations consider the field effects from neighboring species even without any orbital (chemical) interactions. Such field effects may profoundly impact the orbital energies of all of the involved moieties. This is achieved with our block-localized wave function (BLW) method that is the simplest variant of ab initio valence bond (VB) theory and can self-consistently derive a hypothetical diabatic state where the species stay physically together but exclude chemical interactions. Case studies of an exemplary dative bond in H₃B-NH₃ and an unconventional ionic bond in lithium-aluminum dimetallocenes demonstrate that the novel "<i>in situ</i>" orbital correlation diagram not only provides more insight than the traditional one in general cases but also reshuffles the orbital correlations in cases where the traditional orbital correlation diagram fails.