AB-INITIO CALCULATIONS OF ELECTRIC-FIELD GRADIENTS IN CADMIUM COMPLEXES

Calculations of the electric field gradient (EFG) at the cadmium nucle us have been carried out on Cd2+ in the field of two point charges, Cd F2, CdCl2, and CdF2Cl22- at the RHF, MPn (n = 2, 3, 4), and CCSD(T) le vels of theory, in order to evaluate the effects of electron correlati on, relativity, and basis set truncation. The EFG has furthermore been calculated in two large molecules (approximately 300 electrons) with biologically relevant cadmium ligands. Different methods to truncate t he system have been investigated. The results are compared to experime ntal values determined in polycrystalline samples. We suggest a reason ably accurate and economic procedure to calculate the EFG on large cad mium complexes. The basis set on cadmium should be large, at least [19 s15p9d4f/11s9p5d2f], while 6-31G(d) can be used on the remaining atoms . Correlation should be treated at least at the MP2 level, which is fo und to be unexpectedly accurate due to cancellation of higher order te rms. In this treatment the core orbitals on the ligands and 1s through 3d orbitals on cadmium can be frozen. Surrounding molecules in the cr ystals have been modeled by an array of point charges. Using this proc edure, the error of the elements of the diagonalized EFG tensor is les s than 0.3 au (3 x 10(21) V/m(2)) for the investigated complexes.