Counterpoise-corrected potential energy surfaces of simple hydrogen-bonded systems

Geometric and energetic characteristics of various simple hydrogen-bonded complexes (water dimer, hydrogen fluoride dimer, formamide dimer, formic acid dimer, glycine dimer) have been studied by gradient optimization, which a priori eliminates the basis set superposition error (BSSE) by using the counterpoise (CP) method, as well as by the standard gradient optimization. Calculations were done at the Hartree-Fock, correlated MP2 and DFT levels with small- and medium-basis sets. The CP-corrected and standard PESs differ, depending on the theoretical level used. Larger differences were found if the correlation energy was included. Intermolecular distances from the CP-corrected PES are consistently longer, and the respective difference may be significant (approximate to 0.1 Angstrom). The stabilization energies obtained from the CP-corrected PES are always larger than those from the standard PES. Optimization at the standard PES might result in a wrong structure. For example, the "quasi-linear" structure of the (HF), (global minimum) does not exist at the standard MP2/6-31G** and DFT/B3LYP/6-31G** PESs and it is found only when passing to the respective CP-corrected PESs.