Valence shell calculations on polyatomic molecules

Molecular quadrupole moments were calculated for some diatomic, symmetric top and small planar molecules by CNDO/2D and SCC methods. The results resemble those obtained by ab initio minimal basis set calculations and give a fairly good agreement with experimental data. The latter are closely reproduced only by ab initio extended basis set calculations indicating that molecular quadrupoles provide a sensitive test of the quality of the molecular wave functions. Flygare's empirical additivity rules for out-of-plane second moments of electronic charge distribution were rationalized in terms of CNDO/2 MO's over orthogonal AO's. In this model only one-centre terms are important and the sum of one-centre terms was approximately constant for the first-row atoms.