REFORMULATION OF THE LDA-ORBITAL BASIS(U METHOD FOR A LOCAL)

We present a local orbital approach to the evaluation of the on-site r epulsion energy U for use in the LDA+U method of Anisimov and co-worke rs. Our objectives are to make the method more firmly based, to concen trate primarily on ground-state properties rather than spectra, and to test the method in cases where only modest changes in orbital occupat ions are expected, as well as for highly correlated materials. Because of these objectives, we employ a differential definition of U. We als o define a matrix U, which we find is very dependent on the environmen t of the atom in question. The formulation is applied to evaluate U fo r transition-metal monoxides from VO to NiO using a local-orbital basi s set. The resulting values of U are typically only 40-65 % as large a s values currently in use. We evaluate the U matrix for the e(g) and t (2g) subshells in paramagnetic FeO, and illustrate the very different charge responses of the e(g) and t(2g) states..The sensitivity of the method to the choice of the d orbitals, and to the basis set in genera l, is discussed. [S0163-1829(98)05427-7].