We have calculated the interaction energy of two HF molecules at the single‐configuration Hartree–Fock level using 34 different basis sets in an effort to assess the reliability and usefulness of the counterpoise correction to account for basis set incompleteness. We find large counterpoise corrections for all configurations studied, and we show that using a large enough basis set so that the counterpoise correction is small does not guarantee accurate results. Furthermore even for smaller basis sets the inclusion of counterpoise corrections does not systematically improve the accuracy of the calculations.

Systematic study of basis set superposition errors in the calculated interaction energy of two HF molecules

Weakly Bonded Systems

A perturbational method is described for calculating the interaction energy of two molecules in the region where the overlap between their wave-functions is significant. By working directly with a basis of determinants constructed from the SCF orbitals of the separated molecules, without orthogonalization, it is possible to avoid many of the disadvantages of other methods.

An intermolecular perturbation theory for the region of moderate overlap

We have performed a study of the water dimer interaction using larger basis sets and higher levels of theory than have been previously applied to this system. For the minimum geometry we have used spdf basis sets containing up to 212 orbitals. Our most accurate SCF interaction energy for the minimum is −3.73±0.05 kcal/mol. We have shown that this energy can be reproduced to within 0.1 kcal/mol using much smaller basis sets containing proper (diffuse) exponents. Accounting for the basis set superposition error is shown to be essential. We computed the dispersion energy with neglect of the intramolecular correlation using basis sets of various sizes. The best value obtained in a large spdf basis set with exponents which optimize this quantity is −1.93 kcal/mol and it is expected to be accurate to 0.1 kcal/mol or better. Using some of these basis sets we have performed supermolecular many‐body perturbation theory (MBPT) and coupled‐cluster (CC) calculations including triple excitations. We have shown that if...

D.L. Merrifield

Papers

Ghost orbitals and the basis set extension effects