Recently, a novel type of intermolecular covalent interaction has been found in dimers with π-radical(s). In this paper, the optimized structure of the tetrathiafulvalence radical-cation dimer (TTF·⁺–TTF·⁺) is obtained with all-real frequencies, in which a 20-center-2-electron intermolecular covalent π/π bonding with a double-tube shape is theoretically predicted. The covalent π/π bonding energy is estimated to be about -21 kcal·mol^-1 which counteracts partly the Coulombic repulsion between two TTF·⁺ cations. This intermolecular covalent bonding can also influence the structure of the TTF·⁺ subunit, i.e., its molecular plane is bent by an angle θ = 5.6°. In addition, adding the background charges based on the TTF-TCNQ crystal structure, the interaction energy of TTF·⁺–TTF·⁺ changes from a positive value to a large negative value.

This work indicates that the TTF-TCNQ crystal structure built up from parallel, segregated stacks of cation radicals TTF·⁺ and anions TCNQ·^- is stabilized by two main factors: the intermolecular covalent π/π bonding interaction between like-charged radicals and the effect of counterions.