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It is generally accepted that the properties of the high-temperature phases require a reduction in bonding coming from 5f electrons, but the source of this reduction has not been determined. Strong electron correlations and localization driven by structural distortion are two of the suggestions that have been put forward. Attempts to include electron correlation beyond the local density approximation have been limited to phenomenological Hamiltonians lacking sufficient accuracy to deal with, and be tested against, real materials and structures. At the present time, the size of a unit cell necessary for a realistic study of disorder effects is prohibitively large. We have developed a model, which we call the mixed-level state, that relies only on the assumption that partial localization of the 5f manifold is responsible for the reduced bonding. The mixed-level model is quite successful in describing the basic structural properties of delta Pu and Pu-Ga stochiometric alloys. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]