BASIS SET SUPERPOSITION ERRORS IN INTERMOLECULAR STRUCTURES AND FORCE-CONSTANTS

It is now generally accepted that one has to correct the energies for the basis set superposition error in the ab initio calculation of intermolecular complexes. However, we call into question the commonly used procedure (at least for weak complexes with only small electrostatic interactions), where, after a normal gradient optimization, the energy is corrected in a single-point calculation. We investigate the influence of the counterpoise correction on the intermolecular structure and force constants in weak complexes. With a 6-31G** basis set, intermolecular distances may be wrong by 30 to 50 pm and frequencies by a factor of 2 to 4.