The “Perfect” Form of Heitler-London Theory
One of the many reasons behind the unwillingness of chemists to explore VB theory as a qualitative theoretical tool is the apparent size of the CW basis set. For example, in the case of a 4e-4c system, a complete VB basis set includes just two HL CW’s but eighteen more CW’s of the “ionic” type. However, this prolem can be circumvented by replacing localized by delocalized AO’s and implementing HL theory over such a basis. This brand of HL theory is the implicit form of VB theory with a complete basis of CW’s constructed from localized AO’s. The only difference between this type of HL theory and the traditional VB theory is conceptual accessibility, i.e., explicit VB theory makes more chemical sense than implicit VB theory. In order to understand how this apparently unanticipated equivalence arises, it is necessary to consider the elementary problem of two electron-two center bonding, where the two orbitals are denoted by x1 and x2. The following discussion draws from important arly contributions by Coulson and Fischer 92and Slater during the initial phase of development of VB theory.
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