History PostWWII and Automatic Computation

The period during and about ten years after WWII saw the beginnings of the development of automatic computing machinery. Although early workers made heroic efforts in many calculations, computers allowed calculations of molecular structure that were far too tedious to undertake by hand or to expect reliable results. The new computers thus allowed many of the quantitative procedures worked out earlier to be checked and accepted 

Coulson and Fisher[20] took a new step in molecular calculations with then-treatment of H2 in which the orbitals were non-orthogonal, but extended over both centers. They do not actually call their treatment a VB calculation, but their idea is an important step in the development of the ideas of others who do use the VB label in describing their treatments. 

The essence of this method, when illustrated with H2, is to write the two orbitals for the covalent Heitler-London function as 

The constant c provides a parameter to vary during optimization. They, in effect, used molecular orbital (MO)s in the wave function, but this terminology is not usually used in the current context. The introduction of this sort of orbital provides the same effect as ionic terms in the more traditional treatment. The next two sections give modem extensions of this method. 

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