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Ab initio calculations of electric field gradients in cadmium complexes

Lars Hemmingsen & Ulf Ryde

*J. Phys. Chem. 100 * (1996),4803-4809
Calculations of the electric field gradient (EFG)
at the cadmium nucleus have been carried out on Cd^{2+} in the
field of two point charges, CdF_{2}, CdCl_{2},_{ }and
CdF_{2}Cl_{2}^{2-} at the RHF, MP*n *(*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 ligand and 1s through 3d orbitals on cadmium can be frozen.
Surrounding molecules in the crystals have been modelled 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 10^{21} V/m^{2}) for the investigated complexes.