Theory of Pair Functions

Theory of Pair Functions.—We now turn to the model in which the electronic state of a molecule is represented by wavefunctions of the form 

The energy given by a function of the form (98) may be expressed as88 

The use of the Serber basis for spin functions combined with the strong orthogonality condition (100) ensures that separated pair functions (98) 

The most common approach is to note that since the pair functions vv/ may not contain any interparticle co-ordinates (as then the strong orthogonality requirement cannot be sustained), they may be expressed as bilinear expansions in one-electron orbitals  

The orbitals themselves are further expanded in a basis set of atomic orbitals 

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