Molecular orbital studies of TTF and TCNQ dimers by the extended Hückel method

Total energy calculations are presented for (TTF)2 and (TCNQ)2 dimers for various relative positions and orientations of the molecules with the hope of obtaining some insight into the way that these molecules stack in the solid. It is found that full charge transfer (modeled by the doubly charged dimer) favors that stacking with one molecule directly above the other (although the slipped configuration is also favorable for TCNQ), while for charge transfers nearer to half an electron per molecule the molecules tend toward a slipped configuration with a relative displacement along the long axis of the molecules. The problem of relating dimer splittings to band widths is briefly discussed.