Semiempirical versus ab Initio Calculations of Molecular Properties. I. Diamagnetic Susceptibilities and Quadrupole Moments in Some Medium Size Molecules

Abstract

The self-consistent charge molecular orbital (SCC-MO) method was applied to a number of medium size molecules involving atoms exhibiting large differences in electronegativity and possessing one or more lone pairs. The quality of the wave functions thus obtained was applied to a number of medium size molecules involving atoms calculated by a rigorous treatment of the operators x2, y2, and z2• The SCC quadrupole moments can be favourably compared with the ab initio results of Snyder and Basch obtained by the use of a Gaussian double zeta basis set. The present results provide additional justification of the sec method, which has apparent advantages over the methods based on the ZDO approximation. Some difficulties encountered in the treatment of pi-electrons are discussed and suggestions for their remedy are given. The second moments calculated by the application of a simple additivity formula (Z. B. Maksic and J. E. Bloor, Chem. Phys. Lett., 13 (1972) 571; J. Phys. Chem., 77 (1973) 1520) and the related diamagnetic susceptibilities are in very good agreement with the ab initio values