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 apriori 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 harmonic vibrational frequencies twelve-dimensional anharmonic frequencies were also determined using the second-order perturbation treatment. Harmonic and anharmonic frequencies were based on CP-corrected Hessians. The equilibrium geometry of the dimer differs from that determined by a standard optimization and the difference becomes small only for the largest basis set (aug-cc-pVQZ). The best theoretical estimate of the intermolecular oxygen–oxygen distance (2.92 A) is shorter than the experimental result of 2.95 A. An estimate of the complete basis set limit of the stabilization energy was obtained by extrapolating the stabilization energies as a function 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 due to zero-point energy and temperature-dependent enthalpy terms (determined using anharmonic frequencies obtained from the CP-corrected Hessian) we obtain 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 results. 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 with the experimental value was found for anharmonic frequencies calculated with CP-corrected Hessians.