COMPARISON OF DENSITY-FUNCTIONAL AND MP2 CALCULATIONS ON THE WATER MONOMER AND DIMER

The geometries and vibrational frequencies of the water monomer and dimer are calculated using the MP2 method and density functional theory (DFT). The DFT calculations are carried out using both the Becke-Perdew and B3LYP nonlocal exchange-correlation functionals, with the latter being comprised of Becke's three-parameter exchange functional [J. Chem. Phys. 1993, 98, 5648] and the Lee-Yang-Parr correlation functional [Phys. Rev, B 1993, 37, 785]. DFT calculations with the B3LYP functional, in contrast to those with the Becke-Perdew functional, are found to give geometries and frequencies in close agreement with the MP2 and experimental results. The B3LYP calculations with the largest basis set employed give a dimer binding energy of -4.56 kcal/mol, about 0.5 kcal/mol smaller in magnitude than the binding energy obtained from MP2 calculations with similar basis sets.