Ab initio calculations of static dipole polarizabilities using improved virtual orbitals and symmetry adapted polarization functions

Abstract

The method previously described by the present authors [1] for the calculation of molecular static electric dipole polarizabilities is applied to a series of saturated and unsaturated compounds. The method is based on the use of improved virtual orbitals within a sum over states (SOS) formalism. The basis sets used are a large basis of double and triple zeta quality including diffuse functions and two sets of polarization functions ([DTZ/DPP]) and a reduced version of it in which only those polarization functions which are needed by symmetry are included. The construction of this basis set which is obviously symmetry dependent is described for the molecules studied. The use of improved virtual orbitals considerably improves the results for the ten-electron molecules HF, NH₃ and CH₄, in line with the H₂O case study in [1]. The results obtained with the largest basis are comparable with those obtained by more elaborate, beyond Hartree-Fock, Finite Field calculations. The reduced basis performs almost equally well and forms an interesting alternative when the large [DTZ/DPP] basis becomes untractable. For unsaturated systems (C₂H₄, H₂CO, CH₃CN, N₂, CO₂, HCN and C₂H₂) the component parallel to the double or triple bond is overestimated in the IVO technique. Fair agreement with experiment is however obtained for the perpendicular component(s). The deviation from experiment increases with increasing polarity, and degree of unsaturation. In the case of linear molecules considerable improvement can be obtained via the introduction of symmetry breaking spherical gaussian orbitals or by slightly bending the molecule.