Assessing Relativistic Effects and Electron Correlation in the Actinide Metals Th to Pu

• Main Authors: Babak Sadigh, Andrey Kutepov, Alexander Landa, Per Söderlind
• Format: Article
• Language: English
• Published: MDPI AG 2019-11-01
• Series: Applied Sciences
• Subjects: density functional theory; actinide elements; spin–orbit interaction; electron correlation
• Online Access: https://www.mdpi.com/2076-3417/9/23/5020

Density functional theory (DFT) calculations are employed to explore and assess the effects of the relativistic spin&#8722;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&#8722;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&#8722;orbit (SO) interaction for the p_1/2 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&#8722;orbit coupling, orbital polarization, and non-collinear magnetism, while retaining an efficient calculation of Hellmann&#8722;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.