  We present a new approach to the evaluation of the on-site repulsion energy U
for use in the LDA+U method of Anisimov and collaborators. Our objectives are
to make the method more firmly based, to concentrate primarily on ground state
properties rather than spectra, and to test the method in cases where only
modest changes in orbital occupations are expected, as well as for highly
correlated materials. Because of these objectives, we employ a differential
definition of U. We also define a matrix U, which we find is very dependent on
the environment of the atom in question. The formulation is applied to evaluate
U for transition metal monoxides from VO to NiO using a local orbital basis
set. The resulting values of U are typically only 40-65% as large as 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 response 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 general, is discussed.

Strongly Correlated Electrons;Materials Science