Following preliminary calculations that revealed the need to consider electron correlation and to avoid the basis set superposition error, MP2/6-311+G* calculations were carried out in order to determine the energy of intermolecular interaction for 336 configurations of dimeric acetonitrile, to which an analytic potential function was fitted. The minimum of the function (-20.8 kJ mol(-1)) corresponds to an antiparallel geometry with C-2h symmetry where the C-H bonds in the symmetry plane are oriented toward the nitrogen atom of the other molecule. Rotating the methyl group 60 degrees affords a transition state with an energy 1.7 kJ mol(-1) above the minimum. Several minima for the trimer and tetramer were also characterized that revealed a tendency of the molecules to retain the antiparallel arrangement.