Singlet ground state, HPHF function

THE HPHF FUNCTION FOR THE SINGLET GROUND STATE 2.1. The Brillouin Theorem... 

The HPHF function is indeed not a pure spin function, but, in the ground state the wave function for singlets will not contain any triplet component which is usually the largest contaminant in the DODS function. Furthermore, it may be expected that, in the ground state, the quintuplet contamination will be very small because of its high energy. Different procedures have been developed and applied successfully to small molecular systems [8-11]. A review on this subject is given in Ref.[12. ... 

In addition, the two-determinantal form of the HPHF function suggests the use of this model for the direct determination of the lowest singlet excited states. In this aim, a procedure similar to that for the ground state was developed and successfully applied to small molecular systems [13-16]. In these calculations, the HPHF model was shown to yield much better results than single excitation Cl calculations [15],... 

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