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

Babak Sadigh,Per Söderlind,Andrey Kutepov,Alexander Landa

doi:10.3390/app9235020

Babak Sadigh, Per Söderlind + Show 2 more

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https://doi.org/10.3390/app9235020

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Abstract

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 U parameter (DFT + U). Contrary to recent beliefs, we show that for the ground-state properties of the light actinide elements Th to Pu, the DFT + U makes its best predictions for U = 0. Actually, our modeling suggests that the most popular DFT + U 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 p1/2 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–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.

Highlights

Our modeling suggests that the most popular Density functional theory (DFT) + U formulation leads to the wrong ground-state phase for plutonium
We 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 show results derived from density functional theory that was developed in the mid-1960s [42,43]

Summary

Introduction

In the last several decades, various computational techniques to calculate the electronic structure and ground-state properties of actinides have been suggested in the literature [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26] In these studies, a comparison of the calculated equilibrium volumes with experiments has often served the purpose to evaluate the quality of the modeling and the underlying physics of the 5f electrons. The physical interpretation, given by Friedel [27], is that for the first half of the series, bonding electrons contract the volumes while for the second half, anti-bonding electrons expand the lattice.

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