Geminal product wavefunctions

GVB (PPGVB) wavefunction. This wavefunction form was first proposed by Hurley et who derived the energy expression directly in terms of the parameters A. These authors arrived at this wavefunction form, however, as a special case of a geminal product wavefunction form. The PPGVB wavefunction form has been used extensively by Goddard and coworkers ° ° . The wavefunction for N electrons (consisting of N/2 pairs) may be written as... 

An AGP wavefunction [48, 49] is generated from wedge products involving a single geminal g(l,2) ... 

Finally, one other non-linear wavefunction expansion will be described. If the geminals of the different electron pairs are further restricted to be identical for each electron pair, then the result is called the antisymmetrized product of identical geminals (APIG) wavefunctionThere are only (n—1) parameters in the APIG wavefunction which spans a subspace of the GP wavefunction space. Because of the severely restrictive form of this wavefunction, it has not been used extensively for MCSCF calculations but it has been used as a reference function for propagator calculationsfor which this wavefunction form has formal appeal. 

The electron-pair bond plays a central role in the qualitative understanding of molecular structure. The pair-function or geminal approach attempts to put this electron-pair concept into a more quantitative form. In this approach the total wavefunction is assumed to be of the form of an antisymmetrized product of pair-functions ... 

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