Quasi-bond orbitals from maximum-localization hybrids for ab initio CI calculations

A method is presented for localizing canonical HF molecular orbitals to obtain physically meaningful orthogonal ‘quasi-bond orbitals’ (QBO). Atomic hybrids and nonorthogonal bond orbitals (BO) are defined according to a well-established procedure. The transformation of the HF MO is such that the resulting QBO resemble as closely as possible the BO. With respect to the BO, the QBO have the advantage of being orthogonal and optimized at the HF level. Tests run with a selected CI algorithm (CIPSI) on the two anions [CH2NH2CH2]− and [NH2CH2CH2]− show how convenient it is to use QBO in CI calculations. The resulting CI wavefunctions lend themselves to a clear physical interpretation.