Ab initio calculations on weakly bonded systems

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Abstract
Large-basis CI calculations are performed on the van der Waals complexes Ar–HCl and (H2O)2. It is shown that a reasonable estimate of the CI basis-set superposition error is obtained from a ghost calculation involving the orbitals of the monomer and only the virtual orbitals of the ghost. Both basis-set superposition error corrections and size-consistency corrections are of vital importance to obtain a reliable potential-energy surface. For Ar–HCl the minima of the potential are predicted within 50 µhartree of the experimental surface, viz. –804 µhartree for the Ar–HCl orientation and –565 µhartree for the HCl–Ar geometry. The water dimer van der Waals minimum is estimated to be –4.9 kcal mol–1, which is less deep then experimentally derived minimum of –5.4 kcal mol–1, but just within the experimental error limit.

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