Xαscattered-wave calculations for dimers and trimers of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ)

Abstract

$X\alpha$ scattered-wave molecular-orbital (MO) calculations have been carried out for dimers and trimers of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) in the slipped $C_{2h}$ geometry found in crystalline TTF-TCNQ. The MO's of the dimers which are the precursors of the partially filled valence and conduction bands in TTF-TCNQ are found to a good approximation to be simply bonding and antibonding combinations of the highest occupied MO (donor level) in the case of TTF and the lowest unoccupied MO (acceptor level) in the case of TCNQ. When extended basis sets are used, the energy difference between corresponding bonding and antibonding levels (dimer splitting) is 0.24 eV for a TTF dimer and 0.29 eV for a TCNQ dimer. These basis sets include $s$, $p$, and $d$ partial waves on C, S, and N atoms, and $s$ and $p$ partial waves on H atoms. When the dimer and trimer levels are fitted to a tight-binding model containing the transfer integral $t$ and the overlap integral $S$, it is found that $S$ is negligible. The infinite-stack bandwidths $W$ are then equal to twice the dimer splittings and the transfer integrals to half the dimer splittings. The derived TTF and TCNQ bandwidths (0.48 and 0.58 eV, respectively) are consistent with available experimental estimates.