A recently developed local approach to the computation of electron-correlation energies is extended and investigated in great detail. For that purpose it is applied to the Ne atom and the CH. molecule for which correlation-energy calculations are available. It is shown that the local approach exhausts up to a few percent the correlation energy which is obtainable within a given basis set. A detailed comparison with conventional CI methods reveals the advantages of the present approach. Only a fraction of the twoelectron matrix elements Vijkl has to be calculated, and the number of required configurations is drastically reduced. Furthermore, CH. is used as an example to demonstrate that the correlation-energy calculations for large systems can be broken up into ones for small systems.

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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.