EFFECT OF THE BASIS-SET ON THE COMPUTATION OF MOLECULAR-POLARIZATION

The effect of the basis set on the polarization contribution to the interaction energy was studied by means of the polarization energy maps for hydrogen cyanide, formaldehyde, and urea. The study was carried out at the SCF level. Basis sets included in the present study were the split-valence 3-21G, 6-31G, 6-31G*, and 6-31G** basis sets and Dunning's 9s5p contracted to a double zeta (DZ), the DZ plus one set of polarization functions (DZP), or two (DZPP'). A DZ plus one set of polarization functions with exponents optimized to yield reliable polarizabilities (DZP') was also used. The polarization at each point was calculated as the difference between the interaction energy of the molecule with a positive point charge and the molecular electrostatic potential at this point. Maps were computed for three different planes at 2, 4, and 8 bohrs from the plane containing the heavy atoms of the molecule. Comparison of the results shows a correlation between the percentage of the polarization achieved with the ability of a basis set to provide reasonable estimates of dipole polarizabilities. Thus, a basis set with polarization functions optimized to obtain reliable polarizabilities was necessary to achieve good quantitative descriptions of molecular polarization potentials.