Reliable theoretical treatment of molecular clusters: Counterpoise-corrected potential energy surface and anharmonic vibrational frequencies of the water dimer

Structure, properties and energetics of the water dimer were determined by counterpoise (CP)-corrected gradient optimization which a priori eliminates the basis set superposition error (BSSE). Calculations were carried out at the MP2 level with various basis sets up to the aug-cc-pVQZ one. Besides h armonic vibrational frequencies twelve-dimensional anharmonic frequencies w ere also determined using the second-order perturbation treatment. Harmonic and anharmonic frequencies were based on CP-corrected Hessians. The equili brium geometry of the dimer differs from that determined by a standard opti mization and the difference becomes small only for the largest basis set (a ug-cc-pVQZ). The best theoretical estimate of the intermolecular oxygen-oxy gen distance (2.92 Angstrom) is shorter than the experimental result of 2.9 5 Angstrom. An estimate of the complete basis set limit of the stabilizatio n energy was obtained by extrapolating the stabilization energies as a func tion of the reciprocal size of the basis set; this value (21.05 kJ mol(-1)) is slightly smaller than other literature estimates. Adding the changes du e to zero-point energy and temperature-dependent enthalpy terms (determined using anharmonic frequencies obtained from the CP-corrected Hessian) we ob tain an estimate to the theoretical stabilization enthalpy at 375 K (12.76 kJ mol(-1)) which is by 0.8-1.3 kJ mol(-1) smaller than the literature resu lts. Our theoretical value supports the very low limit of the experimental value. Red shift of the O-H stretching frequency accompanying formation of the dimer was determined at various theoretical levels and best agreement w ith the experimental value was found for anharmonic frequencies calculated with CP-corrected Hessians.