Abstract
The two-electron-self-consistent-field (SCF2) approach in which the wave function is constructed from spin-unrestricted geminals (UG's) optimized variationally is developed. It combines restricted-geminal (RG)-SCF2 and unrestricted Hartree-Fock methods, thus accounting for both short-range (intrapair) and long-range electron correlations. On this basis ground-state properties of the extended half-filled Peierls-Hubbard chains are calculated in a wide range of site-energy modulation \ensuremath{\alpha} and on-site electron-electron repulsion U. Treating a generalized-Peierls transition in slight-to-strong correlation regimes within the same framework, the electronic-Peierls and spin-Peierls transition areas can be conventionally differentiated by bifurcation points in which UG-SCF2 solutions separate from RG solutions. The corresponding phase diagram in the U-\ensuremath{\alpha} plane is obtained. For the monatomic (\ensuremath{\alpha}=0) chain, the predicted dimerization amplitude is in a good agreement with the earlier Peierls and spin-Peierls calculations. For the diatomic strongly correlated chain, dimerization sharply rises with \ensuremath{\alpha} at the bifurcation point ${\mathrm{\ensuremath{\alpha}}}_{0}$ and then rapidly drops near \ensuremath{\alpha}=U/2; a property that can be related to phase transitions in some donor-acceptor molecular crystals.
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