A POINT-CHARGE MODEL FOR THE NUCLEAR-QUADRUPOLE MOMENT - COUPLED-CLUSTER, DIRAC-FOCK, DOUGLAS-KROLL, AND NONRELATIVISTIC HARTREE-FOCK CALCULATIONS FOR THE CU AND F ELECTRIC-FIELD GRADIENTS IN CUF

A point charge model for the nuclear quadrupole moment tensor (PCNQM) is developed in order to determine accurate electric field gradients ( EFG at the relativistic and correlated levels. The symmetric s contrib utions arising from the Poisson equation are avoided by using an appro priate point charge distribution in three-dimensional space. It is sho wn that the PCNQM model yields virtually the same EFGs compared to the conventional method of expectation values, if the paint charges are s et at small displacements from the nucleus (d < 10(-13) m) and the SCF energy is converged out to 12 significant figures. We further demonst rate that the choice of the point charge zeta is not very critical to the PCNQM perturbation, and that the correlation energy at both the no nrelativistic and relativistic level of theory depends linearly on zet a. This suggests that accurate EFG tensors can be obtained by performi ng only two correlated calculations for each atom and tensor component . The PCNQM model is tested on one-electron atoms and on the Cu and F EFG in CuF. Relativistic and correlation effects on EFGs are discussed in detail. A Z-expansion on one-electron systems demonstrates that th e relativistic correction scales similar to Z(5). For the CuF molecule Douglas-Kroll and Dirac-Fock coupled cluster calculations an in good agreement with each other. At the best level of theory (coupled cluste r Dirac-Fock plus correction from basis set incompleteness) we obtain a nuclear quadrupole coupling constant for Cu-63 of 23 Mhz. This is in very good agreement with the experimental result of 22 MHz considerin g the large standard deviation of the Cu-63 nuclear quadrupole moment applied, 220(10) mb. (C) 1998 American Institute of Physics.