INFLUENCE OF POLARIZATION FUNCTIONS ON ATOMIC PROPERTIES OF BRIDGE N-ATOM AND H-ATOM OF HCN-CENTER-DOT-CENTER-DOT-CENTER-DOT-HCN

The effect of different types of core and valence polarization functio ns on the equilibrium geometry, electric fields, Cioslowsky and Mullik en atomic charges, dissociation energy and C-H stretching frequency ch anges 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 ac curate basis sets, Calculations of equilibrium geometry, total energie s, harmonic frequencies and electric fields of some test molecules sho wed excellent agreement with accurate results. The resulting basis set s were considerably large in size and the need for reliable calculatio ns of the hydrogen bonds of the HCN dimer with a 6-31G basis set using different procedures to include polarization functions was investigat ed. 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 leve l of theory for the HCN dimer. The agreement between the properties ca lculated with the polarized 6-31G basis set and the standard 6-GCM bas is was analyzed using the simple statistical Euclidean distance, Compa rison of the Euclidean distances for molecular properties (electric fi eld, Cioslowsky charges, bond lengths, change in C-H stretching freque ncy and dissociation energies) suggests that 6-31G polarized with the Pople's original d type functions or mixing Pople's polarization funct ion 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 im proved, other molecular properties are also better characterized. Eucl idean distances for Mulliken charges have shown a different tendency, If only Mulliken charges are considered, the polarization of hydrogen is sufficient.