The Effect of Basis Set Superposition Error (BSSE) on the Convergence of Molecular Properties Calculated with the Correlation Consistent Basis Sets

It is shown that, in many cases, the convergence behavior of molecular properties computed with the correlation consistent basis sets (both standard and augmented sets) is significantly improved if basis set superposition error (BSSE) is taken into account. The effects are most pronounced for pure van der Waals systems like the helium or argon dimers. For these systems the uncorrected D_e, r_e, and ω_e behave very irregularly with increasing basis set size, with the convergence behavior being dramatically improved by use of the counterpoise procedure. Even for strongly bound diatomics like N₂, HF, and HCl, the counterpoise correction often significantly improves the convergence behavior of r_e and ω_e. Similar behavior is observed in the weakly bound molecular complexes, ArHF, HCO^-, and (HF)₂, as well as for the more strongly bound HCO molecule. For HCO^-, because of the pronounced lengthening of the CO bond upon molecular formation, the deformation energy must also be taken into account.