THE ELECTRIC-FIELD GRADIENT AT THE BE-9 NUCLEUS IN BEC

The electric field gradient q at the Be-9 nucleus in BeC (X 3SIGMA-) i s computed as a function of the internuclear distance using the config uration interaction methodology and and natural orbitals constructed o ver an extended atomic basis set of 117 contracted Gaussians. In contr ast to molecules like LiH and BeH+, with no pi bonding, and for which correlation effects account for only approximately 2% of the value of q, in BeC, with its two unpaired pi electrons, the major contribution to q (approximately 97%) is described properly only with a highly corr elated wavefunction. As in LiH and BeH+, this field amounts on the ave rage to approximately 80% of the atomic case (Li(P-2) and Be(P-3)). Th e effect of vibration on the field gradient, computed as an averaged v alue of q(R) over the vibrational states, shows a variation of approxi mately 2% with increasing v; the effect of rotation is practically neg ligible up to J = 5.