Electrostatic interactions in tetrathiafulvalenium-tetracyanoquinodimethanide (TTF-TCNQ): Madelung energy and near-neighbor interactions

The Madelung energy of tetrathiafulvalenium-tetracyanoquinodimethanide (TTF-TCNQ) has been calculated as a function of assumed charge transfer, charge distribution, and temperature, using a modified Evjen summing criterion. A large, temperature-independent, Madelung energy, $E_M\approx -2.3\mathrm{}$ eV/molecule (≅ -53 kcal/mole), was calculated for the case of delocalized unit charges on the molecules. $E_M$ alone is shown to be insufficient to stabilize charge transfer in TTF-TCNQ, and the polarization energy is shown to be the most plausible source of additional energy gain upon charge transfer. The near-neighbor Coulomb interactions are evaluated and shown to be the major source of energy gain upon charge delocalization on the molecule.