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The theory of magnetism in transition metals is often expressed in terms of a Stoner exchange parameter I. Results for I are presented from electronic-structure calculations with correlations for a d-band-only Hubbard type of Hamiltonian. The main effects are: (a) Due to correlation, I depends substantially on the volume via the bandwidth. (b) There are analogous corrections to lattice constants calculated with the local-density approximation (LDA). (c) I also depends on magnetization, band filling, and crystal structure. (d) In Ni the reduction of charge and spin fluctuations almost eliminates the contribution to I of exchange between different orbitals. (e) I computed in the LDA is usually overestimated by 10%%uo -20%%uo because the LDA largely neglects spin correlations.

The ground state of the three-band Hubbard Hamiltonian for Cu02 planes of high-T, superconductors is investigated. Correlations between holes are included by a local-ansatz approach which starts from the Hartree-Fock approximation and emphasizes the local character of hole correlations. It is found that the hole distribution within Cu02 planes and the region of stability of an antiferromagnetic {AF) ground state are significantly modified by hole correlations. At the filling of one hole per Cu02 unit, and for realistic parameters, the ground state exhibits an AF long-range order. Taking into account the effect of quantum fluctuations, the magnetic moment amounts to 0.47pw hich agrees very well with the experimental value for La2Cu04. The order parameter decreases with doping and disappears for the doping of 0.06 hole per unit cell. It is found that the nearestneighbor oxygen hopping destabilizes AF ground state which proves the importance of this parameter for quantitative analysis. A favorable comparison between the present results and those obtained within variational Monte Carlo, as well as within the Gutzwiller approximation for a similar model, is presented.