The accuracy of current density functionals for the calculation of electric field gradients: A comparison with ab initio methods for HCl and CuCl

The performance of current density functionals is analyzed in detail for th e electric field gradients (EFG) of hydrogen chloride and copper chloride b y comparison with ab initio methods and available experimental data. The ra nge of density functionals applied shows good agreement with coupled cluste r H and Cl field gradients for HCl, as has been demonstrated previously for other main-group element containing compounds. However, the performance of most density functionals is very poor for the Cu EFG in CuCl (EFG for Cu - 0.44 a.u. at the coupled-cluster singles and doubles with perturbative trip les [CCSD(T)] level, compared to, e.g., +0.54 a.u. at the B-LYP level). Onl y the "half-and-half" hybrid functionals give field gradients with the corr ect sign. The reason for the poor performance of the density functional the ory is analyzed in detail comparing density functional with ab initio total electronic densities rho(r). Due to the conservation of the number of part icles, a change in the valence part of the electron density can lead to cha nges in the core part of the density. Errors in valence electronic properti es like the dipole moment and in core properties like the Cu and Cl EFGs ma y therefore be connected. In fact the errors in both properties show a dist inct linear relationship, indicating that if the dipole moment is correctly described by density functionals, the Cu and Cl EFGs may be accurate as we ll. Furthermore, at the atomic level, electric field gradients are describe d with reasonable accuracy by current density functionals as calculations f or the Cu P-2 excited state and the Cu2+ D-2 ground state show. A compariso n between the different density functionals shows that the incorrect behavi or of the electronic density appears to be mainly due to defects in the exc hange part of the functional. (C) 1999 American Institute of Physics. [S002 1-9606(99)31032-1].