NONORTHOGONAL ORBITALS FOR LOCALIZED-ELECTRONS - I - THE SPIN-COUPLEDWAVE-FUNCTION

The Spin-Coupled wavefunction is investigated for one-dimensional half - and quarter-filled clusters within the framework of the (Extended) H ubbard model. It is shown that in the large U/t limit the ground state may be accurately described by a unique configuration of singly occup ied weakly overlapping orbitals, the coupling of the electrons spin be ing properly described by a fully optimized spin eigenfunction. These orbitals may then be used to define effective spin 1/2 hamiltonians wh ose magnetic coupling integrals are derived from the Spin-Coupled mole cular 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. A ll rights reserved.