Quantifying the importance of orbital over spin correlations in δ-Pu within density-functional theory

Spin and orbital electron correlations are known to be important when treating the high-temperature delta phase of plutonium within the framework of density-functional theory (DFT). One of the more successful attempts to model delta-Pu with this approach [P. Soderlind, Europhys. Lett. 55, 525 (2001); P. Soderlind , Phys. Rev. B 66, 205109 (2002); P. Soderlind and B. Sadigh, Phys. Rev. Lett. 92, 185702 (2004)] has included condensed-matter generalizations of Hund's three rules for atoms, i.e., spin polarization, orbital polarization, and spin-orbit coupling. Here, we perform a quantitative analysis of these interactions relative rank for the bonding and electronic structure in delta-Pu within the DFT model. The result is somewhat surprising in that spin-orbit coupling and orbital polarization are far more important than spin polarization for delta-Pu. We show that these orbital correlations on their own, without any formation of magnetic spin moments, can account for the low atomic density of the delta phase with a reasonable equation of state. In addition, this unambiguously nonmagnetic treatment produces a one-electron spectra with resonances close to the Fermi level consistent with experimental valence band photoemission spectra.