Electric field gradients of transition metal complexes: Basis set uncontraction and scalar relativistic effects

The basis set dependence of density functional theory computed electric field gradients (EFG) in 3d transition metal compounds is assessed. Uncontraction of basis sets at the metal atom is crucial for a correct description of its EFG. Original and revised TPSS functionals and their hybrid variants perform best for our test set of experimental gas phase-determined nuclear quadrupole coupling constants. Recent range-separated hybrid functionals that perform well for the difficult case of CuCl, do not perform well for our larger test set. Relativistic effects for the latter, evaluated using the Douglas-Kroll-Hess and zero-order regular approximations, are found to be small.