Relativistic effects on the equation of state of the light actinides
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0893-JJ01-09.1
Relativistic effects on the equation of state of the light actinides Alex Landa and Per Söderlind Physics and Advanced Technologies Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A. ABSTRACT The effect of the relativistic spin-orbit (SO) interaction on the bonding in the early actinides has been investigated by means of electronic-structure calculations. Specifically, the equation of state (EOS) for the face-centered cubic (fcc) model systems of these metals has been calculated from the first-principles density-functional theory (DFT). Traditionally, the SO interaction in electronic-structure methods is implemented as a perturbation to the Hamiltonian that is solved for basis functions that explicitly do not depend on SO coupling. Here this approximation is shown to compare well with the fully relativistic Dirac treatment. It is further shown that SO coupling has a gradually increasing effect on the EOS as one proceeds through the actinides and the effect is diminished as density increases. INTRODUCTION The physics of the actinide metals is challenging both experimentally and theoretically. Recent progress in experimental techniques has, however, made it possible to study with greater detail the phase diagrams and EOS of these complex metals. This development of the experimental capabilities pushes theory to improve accuracy and explore the effects of various approximations in their implementations. Important steps in this direction, specifically valid for the actinides, were the inclusion of SO coupling [1], the usage of the so-called full-potential (FP) techniques that could properly deal with distorted crystal structures [2], and the introduction of the generalized gradient approximation (GGA) [3,4]. A relativistic formulation of the theory usually begins with the Dirac equation. Within the formalism of the atomic sphere approximation Brooks [1] and Söderlind et al. [5] showed that SO is important for the atomic density and bulk modulus for Np and Pu in particular. Andersen [6] proposed that instead of solving a four-spinor Dirac equation, one could include the largest relativistic effects in a Pauli equation where the smallest relativistic effect, the spin-orbit coupling, is introduced as a perturbation. Later, Nordström et al. [7] realized that the Andersen approach caused inaccuracies for the 6p band states which compromised the quality of calculations. One remedy for this, which seems to yield accurate results but has never formally been tested, is to simply omit the SO coupling on the 6p states [7,8]. In light of the need for accurate theoretical modeling of the actinides and the well known sensitivities of the theory to the treatment of the SO, we have chosen to investigate this issue in detail. We specifically investigate the SO effect (full Dirac, perturbation, or neglect) on the EOS. In order to isolate the dependence of SO coupling, we have chosen to focus only on the fcc crystal structure and not involve dependencies on the actual ground-state structures.
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