Mixing of Degenerate Orbitals— First-Order Perturbations

1 Mixing of Degenerate Orbitals—First-Order Perturbations 

if we assume that S = 0, then lAE l = lAEI and closed-shell repulsion is undone. This may seem an unreasonable situation, but in fact many levels of theory make this approximation. As noted earlier in this chapter, most semi-empirical methods, such as AMI and MNDO, neglect overlap. As such, there is no closed-shell repulsion at these levels of theory, a serious deficiency in some situations. In contrast, for all its limitations, extended Hiickel theory (EHT) does not neglect overlap, and so closed-sheU repulsion survives. As discussed, classical Hiickel theory does neglect overlap. In fact, as previously noted, HMOT can be viewed as perturbation theory with S = 0, Haa = Hbb = a, and Hab = P. 

---