THE WATER DIMER INTERACTION ENERGY - CONVERGENCE TO THE BASIS-SET LIMIT AT THE CORRELATED LEVEL

The water dimer interaction energy and its convergence to the basis se t limit was investigated, with electron correlation effects treated at the level of second order Moller-Plesset perturbation theory (MP2). A NO-type and large uncontracted basis sets were used, spreading over a wide range in size; the biggest set included 1046 functions with angul ar momentum up to (l=7). Core correlation effects were treated accurat ely by augmenting the original valence basis with extended sets of cor e polarization functions. The MP2 dimer interaction energy at the basi s set limit was determined to -4.94+/-0.02 kcal/mol, with a contributi on due to core correlation of -0.04 kcal/mol. Furthermore, based on so me elementary considerations from intermolecular perturbation theory, a simple procedure was devised, which brings the counterpoise correcte d interaction energies of moderate basis set calculations closer to th e basis set limit. The interaction energies so obtained turned out be surprisingly stable with respect to extensions of the basis set. (C) 1 997 American Institute of Physics.