The perfect‐pairing valence bond model for the water molecule

Optimized orbitals for the water molecule have been obtained within the perfect‐pairing valence bond model by an ab inito self‐consistent field calculation. Each split pair is optimized by approximating the remaining electron distribution with a localized doubly‐occupied orbital sea. In addition, the overlaps between orbitals of different pairs are optimized by considering higher order pair–pair interactions. The expansion of the energy in the cumulative order of intrapair orbital splittings converges rapidly. The total energy obtained is 49.0 kcal/mole below restricted Hartree–Fock, thus accounting for 21.0% of the correlation energy. The orbitals are well localized into a 1s core pair on oxygen, two equivalent O–H bond pairs and two equivalent lone pairs on oxygen. The hybridization of these orbitals differs considerably from what is expected on the basis of ordinary chemical intuition, the split lone pair orbitals being sp0.56 and sp1.83 and the bonding orbital centered on oxygen being sp1.84. No artif...