Theoretical study of the electronic structure of [tetrathiafulvalene](2)(2+) dimers and their long, intradimer multicenter bonding in solution and the solid state.

The long, multicenter bonding in tetrathiafulvalenium cation radical dimers, [TTF](2)(2+), observed in both the solid state and in solution, were computationally investigated via B3LYP, CASSCF(2,2), and MCQDPT/CASSCF(2,2) calculations. The stability of [TTF](2)(2+) dimers originates from [TTF](*+)...anion(-) or [TTF](*+)...solvent electrostatic interactions, whose sum exceeds the [TTF](*+)...[TTF](*+) repulsion, in solution, or the sum of the [TTF](*+)...[TTF](*+) and anion(-)...anion(-) repulsions in solid state. MCQDPT/CASSCF(2,2) calculations indicate that the electronic ground state of the [TTF](2)(2+) dimers is a closed-shell single state with non-negligible contribution of the open-shell singlet, as is experimentally observed. This ground state is not well reproduced in B3LYP or CASSCF(2,2) calculations.