O-17 NUCLEAR-QUADRUPOLE COUPLING IN MOLECULAR AND LATTICE SYSTEMS BY AB-INITIO CALCULATION OF ELECTRIC-FIELD GRADIENTS

We present ab initio Hartree-Fock calculations of electric field gradi ents, which are related to experimental determinations of nuclear quad rupole coupling constants. The nucleus of special interest is O-17 but other nuclei in the molecules, especially N-14, H-2 and S-33 are incl uded. The calculations were performed (a) as single molecules either a t the SCF and MP2 correlated levels, (b) as dusters of molecules about a test site, or (c) as lattice calculations computed in the unit cell environment. The basis sets are triple zeta valence plus polarisation in (a). Because of limitations in exponent range in (c), the bases in both (b) and (c), where comparison was sought, are DZ at the SCF leve l.