On the orbital theories in the spin-correlation problems. II. Unrestricted and spin-extended Hartree-Fock theories

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Abstract
A perturbation‐variation treatment is applied to interconnect the unrestricted, projected unrestricted, and spin‐extended Hartree‐Fock wavefunctions with the first order sum‐over‐state perturbation wavefunction. The restricted Hartree‐Fock wavefunction is chosen as a starting point. From these, the interrelations among these theories are clarified not only for the first order correction to the wavefunctions but also for their spin densities. Moreover, the natures of the unrestricted and spin‐extended Hartree‐Fock theories in the spin‐correlation problems are examined in the light of the physical reality of the correlation phenomena in open‐shell electronic systems. These results are ascertained numerically by referring the spin densities calculated by these various methods and obtained from experiments.