C-H center dot center dot center dot O H-bonded complexes: How does basis set superposition error change their potential-energy surfaces?

Geometries, vibrational frequencies, and interaction energies of the CNH .. . O-3 and HCCH ... O-3 complexes are calculated in a counterpoise-corrected (CP-corrected) potential-energy surface (PES) that corrects for the basis set superposition error (BSSE). Ab initio calculations are performed at the Hartree-Fock (HF) and second-order Moller-Plesset (MP2) levels, using the 6-31G(d,p) and D95++(d,p) basis sets. Interaction energies are presented in cluding corrections for zero-point vibrational energy (ZPVE) and thermal co rrection to enthalpy at 298 K. The CP-corrected and conventional PES are co mpared; the uncorrected PES obtained using the larger basis set including d iffuse functions exhibits a double well shape, whereas use of the 6-31G(d,p ) basis set leads to a flat single-well profile. The CP-corrected PES has a lways a multiple-well shape. In particular, it is shown that the CP-correct ed PES using the smaller basis set is qualitatively analogous to that obtai ned with the larger basis sets, so the CP method becomes useful to correctl y describe large systems, where the use of small basis sets may be necessar y. (C) 2000 American Institute of Physics. [S0021-9606(00)30937-0].