Ab initio calculations of electric field gradients in cadmium complexes

Calculations of the electric field gradient (EFG) at the cadmium nucleus have been carried out on Cd²⁺ in the field of two point charges, CdF₂, CdCl₂, and CdF₂Cl₂ ^2- at the RHF, MPn (n = 2, 3, 4), and CCSD(T) levels of theory, in order to evaluate the effects of electron correlation, 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 the system have been investigated. The results are compared to experimental values determined in polycrystalline samples. We suggest a reasonably accurate and economic procedure to calculate the EFG on large cadmium complexes. The basis set on cadmium should be large, at least [19s15p9d4f/11s9p5d2f], while 6-31G(d) can be used on the remaining atoms. Correlation should be treated at least at the MP2 level, which is found to be unexpectedly accurate due to cancellation of higher order terms. In this treatment the core orbitals on the ligands and 1s through 3d orbitals on cadmium can be frozen. Surrounding molecules in the crystals have been modeled by an array of point charges. Using this procedure, the error of the elements of the diagonalized EFG tensor is less than 0.3 au (3 × 10²¹ V/m²) for the investigated complexes.