Counterintuitive orbital mixing

Abstract

In both extended Hückel and ab initio SCF molecular orbital calculations instances of anomalous orbital interaction are occasionally observed. The lower energy combination of two interacting orbitals is found to be the out‐of‐phase, antibonding one. Such cases have been called counterintuitive orbital mixing. We present an argument based on SCF perturbation methodology demonstrating that the counterintuitive orbital mixing effect is not an artifact but a natural consequence of orbital interaction between two orbitals that differ greatly in energy but overlap significantly.