## 1 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. From these works a consistent picture emerges for thorium through neptunium where the 5f electrons can be described as bonding and delocalized (band) electrons. The implication is that the actinide volumes decrease in a parabolic fashion with atomic number for Th to Pu, reminiscent of the non-magnetic d-transition metals. The physical interpretation, given by [PERSON] [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. The parabolic behavior is readily verified by assuming a rectangular electron density of states and the corresponding electron-energy dependence on the volume [27]. Plutonium, on the other hand, deviates slightly from this simple trend. In this case, relativistic effects [4], crystal structure [8; 10], and possible electron correlation effects beyond the local density approximation [9] are suggested as explanations for some of the discrepancy noted between theory and experiments. However, a much-improved description of plutonium is obtained, still within the 5f-band picture, when magnetic effects are addressed [13]. Acknowledging these magnetic interactionsand the relativistic spin-orbit coupling, that represent [PERSON]'s three rules for an atom in the solid, results in very good atomic volumes and energetics not only for \(\alpha\)-plutonium but also for the other phases [14; 15; 16; 17; 18]. Another important realization, directly related to the theoretical atomic volumes, is that the spin-orbit interaction requires explicit incorporation of relativistic p\({}_{1/2}\) orbitals in the partial-wave expansion of the wave functions near the atom. This state, however, is not present in electronic-structure codes that use scalar-relativistic basis functions [19; 20]. Nowadays, all-electron calculations that deal with this issue and account for magnetic effects in plutonium very nicely reproduce the expected atomic volumes for the light actinides, Th to Pu, without any ad hoc intra-atomic Coulomb interactions [24]. Furthermore, within the 5f-band model, the correct ground-state phase is predicted for the light actinide metals, Th to Pu [11; 18]. Contrary to the conclusions from numerous past studies [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], it has been proposed in recent years that one must account for Coulomb repulsion in the electronic structure of the light actinide metals [28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39]. In these reports, a variety of \(U\) parameters are chosen, but a small \(U\) has largely been favored (\(U\)\(\sim\)1 eV). Exceptions are investigations focused on \(\delta\)-plutonium, where larger \(U\)'s have often been adopted (\(U\)\(>\) 4 eV), as has been discussed thoroughly in a recent review paper [40]. Most of these DFT + \(U\) calculations [28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39] utilize plane-wave pseudopotential methods and because the obtained atomic volumes are smaller than expected, or for other reasons, one concludes a need for stronger electron correlation and employment of a Hubbard \(U\). In this context, the improvement of the DFT + \(U\) volumes has been misinterpreted as a need for this type of electron correlation in the modeling, when in reality the situation reflects cancellation of errors. Although this misunderstanding that relates to spin-orbit coupling was discussed to a degree in a previous report [41], it has not been thoroughly addressed in the literature.