Ways of Avoiding Basis-set Superposition Errors

To this time no general agreement has been reached in the literature and the problem of how to determine the basis-set superposition error is considered unsolvable by many. Thus methods have been devised to avoid the BSSE either completely or to a large extent. 

A less rigorous approach is to hope for cancellation of the BSSE and the effect of omitted (polarization) functions on the dispersion energy or for cancellation of the BSSE and the total dispersion energy. The latter is implicitly assumed if the geometry of a van der Waals complex is optimized within the Hartree-Fock framework or when uncorrected small-basis SCF results are preferred based on a comparison with experimental data . The former is utilized if no BSSE correction is applied to Cl or M011er-Plesset perturbation theory results for van der Waals molecules - . 

Finally, in view of the current uncertainty with regard to the proper scheme to correct for BSSE, some authors regard the full counterpoise-corrected and the uncorrected interaction energies as results bracketing the real interaction energy for the basis or the experimental interaction energy - °. The counterpoise method is sometimes only used as an indication for the reliability of the calculation Note, however, that a small BSSE may be due 

Summarizing, since the basis-set superposition error may be large (that is, comparable to the interaction energy) and since it depends in an erratic way upon basis set and geometry, and affects both Cl and SCF calculations and is extremely difficult to avoid without reducing the quality of the calculation, it is of vital importance to arrive at a reliable accepted scheme to correct for it. 

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