A higher level ab initio quantum-mechanical study of the quadrupole moment tensor components of carbon dioxide

The quadrupolarity of carbon dioxide has been studied with higher level ab initio methods. Carbon dioxide exhibits ( - + -) quadrupolarity in all directions and an explanation is provided of the origin of the sign of the diagonal elements Q(ii). The quadrupole moment tensor has been computed using restricted Hartree-Fock theory, second-order Moller-Plesset perturbation theory and quadratic configuration interaction theory. A variety of basis sets have been employed up to basis sets of the type [5s, 4p, 2d, 1f] (23s, sp, 2d, 1f). The quadrupole moment tensor component Q(ii) of carbon dioxide falls in the range between - 18.5 and -20.5 Debye Angstrom. The quadrupole moment tensor components Q(perpendicular to) of carbon dioxide are smaller, ranging from - 14.5 to -15 Debye Angstrom, and they are less sensitive to the choice of the theoretical model. The correlated methods consistently predict an increase of Q(parallel to) while they predict a more modest reduction of Q(perpendicular to). It is for the opposing electron correlation effects on Q(parallel to) and ei that the average values of the diagonal elements, <Q(ii)>, are essentially independent of the method and exhibit only a small variation depending on the basis set. On the other hand, the anisotropy of the quadrupolarity, the quadrupole moment Theta, is affected most by the opposing electron correlation effects on Q(parallel to) and Q(perpendicular to). The accurate reproduction of the measured quadrupolarity Theta = -4.3 Debye Angstrom requires a theoretical model that employs both a good method and a good basis set. The results suggest that the use of second-order Moller-Plesset perturbation theory in conjunction with well-polarized triple-zeta basis sets provides a cost-effective and quite accurate method for the estimation of correlation effects on quadrupole moments. (C) 2000 Elsevier Science B,V. All rights reserved.