THE WATER DIMER - CORRELATION-ENERGY CALCULATIONS

The binding energy of the water dimer at the experimental geometry has been calculated employing the Hartree-Fock (HF) method, Moller-Plesset theories MP2, MP3, and MP4, and the coupled cluster method with double excitations. We have also used configuration interaction wavefunctions both at the HFSDCI (single and double excitations from the HF configuration) and MRSDCI (single and double excitations from multiple reference configurations) levels to calculate the interaction energy. Binding energies within quasi-degenerate variation perturbation theory, linearized coupled cluster method, and the average pair correction model are also calculated. The basis set developed and employed in the study yields -76.0674 au for the HF energy of the water monomer at the experimental geometry. The basis set superposition error for the HF and MP2 models is 0.003 and 0.251 kcal/mol. The MP2 interaction energy has been partitioned into pair energies to give a better understanding of the hydrogen bond. The present study is unable to yield results close to -5.4 kcal/mol, the experimental estimate for the interaction energy. Instead, values ranging from -4.2 to -4.7 kcal/mol have been obtained. On the other hand, very large scale MRSDCI wavefunctions which are almost size consistent yield values as poor as -3.3 kcal/mol. Correlation energy density functional models yield interaction energies in the range of -5.2 to -5.8 kcal/mol. However, when the Hartree-Fock exchange term is replaced with exchange energy density functional models, values from -2.8 to -4.5 kcal/mol are obtained.