Valence Bond Methods Based on Localized Orbitals

thereby performing GVB or SCVB calculations. The orbitals can also be defined by pairs that are allowed to delocalize on only two centers (BDOs), or they can be defined as strictly localized on a single center or fragment (see below). The VBSCF method is implemented in the TURTLE module (now being a part of GAMESS—UK) and in the XMVB package. 

Other multiconfiguration VB methods have also been devised, like the biorthogonal valence bond method of McDouall (35,36) or the spin-free approach of McWeeny (37). For an overview of these methods, the reader is advised to consult a recent review (1). 

3 VALENCE BOND METHODS BASED ON LOCALIZED ORBITALS 

In the case where the fragments A and B are polyatomic (e.g., A = B = H3C, in H3C-CH3) and only the A-B bond is being described in a VB manner, the orbitals that are used to construct the determinants may be FOs that have tails on the adjacent atoms (e.g., the H atoms of a given CH3 group), but remain localized in the sense that an orbital of fragment A has no coefficient on B, and vice versa. In such a case, the description of the A-B bond will still require explicit consideration of ionic and covalent components, and the so-defined FOs will be considered throughout the text as a particular case of localized orbitals. 

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