| Summary: | Density functional theory (DFT) calculations are employed to explore and assess the effects of the relativistic spin−orbit interaction and electron correlations in the actinide elements. Specifically, we address electron correlations in terms of an intra-atomic Coulomb interaction with a Hubbard <i>U</i> parameter (DFT + <i>U</i>). Contrary to recent beliefs, we show that for the ground-state properties of the light actinide elements Th to Pu, the DFT + <i>U</i> makes its best predictions for <i>U</i> = 0. Actually, our modeling suggests that the most popular DFT + <i>U</i> formulation leads to the wrong ground-state phase for plutonium. Instead, extending DFT and the generalized gradient approximation (GGA) with orbital−orbital interaction (orbital polarization; OP) is the most accurate approach. We believe the confusion in the literature on the subject mostly originates from incorrectly accounting for the spin−orbit (SO) interaction for the p<sub>1/2</sub> state, which is not treated in any of the widely used pseudopotential plane-wave codes. Here, we show that for the actinides it suffices to simply discard the SO coupling for the p states for excellent accuracy. We thus describe a formalism within the projector-augmented-wave (PAW) scheme that allows for spin−orbit coupling, orbital polarization, and non-collinear magnetism, while retaining an efficient calculation of Hellmann−Feynman forces. We present results of the ground-state phases of all the light actinide metals (Th to Pu). Furthermore, we conclude that the contribution from OP is generally small, but substantial in plutonium.
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