Non-orthogonal orbitals for localized electrons - I. The spin-coupled wavefunction

The Spin-Coupled wavefunction is investigated for one-dimensional half- and quarter-filled clusters within the framework of the (Extended) Hubbard model. It is shown that in the large U/t limit the ground state may be accurately described by a unique configuration of singly occupied weakly overlapping orbitals, the coupling of the electrons spin being properly described by a fully optimized spin eigenfunction. These orbitals may then be used to define effective spin 1/2 hamiltonians whose magnetic coupling integrals are derived from the Spin-Coupled molecular integrals. The same picture emerges for dimerized quarter-filled chains in an Extended Hubbard model, the SC orbitals taking the form of weakly delocalized dimer orbitals. (C) 1998 Elsevier Science B.V. All rights reserved.