Influence of polarization functions on atomic properties of bridge N and H atoms of HCN … HCN

Abstract The effect of different types of core and valence polarization functions on the equilibrium geometry, electric fields, Cioslowsky and Mulliken atomic charges, dissociation energy and CH stretching frequency changes for HCN … HCN was investigated. Basis sets for C, N and H were constructed (6-GCM) from the 6-31G basis sets to improve the Hellmann-Feynman forces and to describe adequately the asymptotic behavior of the wavefunction in the valence region. This new procedure provided accurate basis sets. Calculations of equilibrium geometry, total energies, harmonic frequencies and electric fields of some test molecules showed excellent agreement with accurate results. The resulting basis sets were considerably large in size and the need for reliable calculations of the hydrogen bonds of the HCN dimer with a 6-31G basis set using different procedures to include polarization functions was investigated. The properties calculated at the Hartree-Fock level of theory (SCF) for the HCN dimer using the 6-GCM basis sets were taken as reference. Three sets of polarization functions derived from the original 6-31G basis set were used with the original 6-31G basis set at the SCF level of theory for the HCN dimer. The agreement between the properties calculated with the polarized 6-31G basis set and the standard 6-GCM basis was analyzed using the simple statistical Euclidean distance. Comparison of the Euclidean distances for molecular properties (electric field, Cioslowsky charges, bond lengths, change in CH stretching frequency and dissociation energies) suggests that 6-31G polarized with the Pople's original d type functions or mixing Pople's polarization function with polarization that improves the Hellmann-Feynman forces are in closer agreement with the calculations using the 6-GCM basis set. If molecular properties more directly related to the hydrogen bond are improved, other molecular properties are also better characterized. Euclidean distances for Mulliken charges have shown a different tendency. If only Mulliken charges are considered, the polarization of hydrogen is sufficient.