Studies in the paired orbital method. I

An extension of the alternant molecular orbital method to nonalternant molecules is presented. The key step involves generation of a set of localized virtual orbitals that are paired to the localized occupied orbitals ( LMO ' S ). Each of the paired virtuals is required to occupy the same region of space as the corresponding LMO by applying a generalization of the Edmiston–Ruedenberg localization criterion. Illustrative calculations are reported for the water molecule. Using a double‐zeta basis, it is found that the new paired orbital ( PO ) method gives approximately 20% of the available correlation energy at R = R e , 1.5 R e , and 2 R e . We speculate that the PO wave function may provide a good initial approximation for a subsequent many‐body perturbation treatment.