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Abstract

The TTF/TCNQ complex with three different mutual geometrical arrangements of the donor TTF (tetrathiofulvalene) and the acceptor TCNQ (tetracyanoquinodimethane) are investigated by means of semiempirical INDO calculations. Some ground state properties of special interest in electron donor-acceptor systems like charge transfer, dipole moment and HOMO/LUMO gap are discussed. The Thouless instability conditions of the Hartree-Fock (HF) wavefunction against orbital fluctuations of the singlet, non-singlet and non-real type are studied. In all complexes the HF solution is singlet and non-real stable but non-singlet instability or near instability indicates the existence of unrestricted HF solutions in the vicinity of the spin paired restricted HF determinant. The numerical findings are rationalized in terms of molecular parameters of donor and acceptor and their interaction. The results are generalized to other electron donor-acceptor complexes or cyclophanes.