The structure and stability of dimers of quaterthiophene radical cations are studied using density functional calculations. Several dimer structures are identified that are metastable in vacuum, in which the cations are stacked coplanarly. Evidence for a weak chemical dimer bonding is found from the binding energy curves and the singlet-triplet splittings. Introducing solvent or solid-state effects by embedding the dimer in a dielectric medium results in stabilizing the dimer, as is observed experimentally. An infinite stack of quaterthiophene cations shows a weak spontaneous dimerization. (C) 2000 American Institute of Physics. [S0021-9606(00)30809-1].