An SCF-CI study of the water dimer potential surface and the effects of including the correlation energy, the basis set superposition error and the Davidson correction

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Abstract
A study at SCF-CI level has been performed on the water dimer, with a [541|31] basis set. The effects of the correlation energy, basis set superposition error (BSSE) and Davidson correction were investigated. Maintaining the experimental water geometry, several sections of the energy surface were calculated. They show only one minimum energy structure, the linear trans dimer, in agreement with experiment. The linear cis and bifurcated forms are found to be unstable. The Davidson correction has a minimal effect on the energy surface. The correlation energy and BSSE are important, being ∼ 10 per cent or more of the total interaction energy, and are distance dependent. They also show a complex angular dependence. As the BSSE size is an indication of the basis set quality, only those surface features virtually unaffected by the BSSE correction can be regarded as reliable. Here, these features are the angular parameters of the minimum, the O-O stretching frequency and the general surface shape. Basis sets with negligible BSSE are desirable, and are at present possible for small systems at SCF level, but not at the CI level [20, 21]. Therefore, current use of this method to provide data for improving empirical energy potentials of systems with non-negligible dispersion energies is unlikely to be successful.