Interplay between London Dispersion, Hubbard U, and Metastable States for Uranium Compounds

Abstract

High-throughput computational studies of lanthanide and actinide chemistry with density-functional theory are complicated by the need for Hubbard U corrections, which ensure localization of the f-electrons, but can lead to metastable states. This work presents a systematic investigation of the effects of both Hubbard U value and metastable states on the predicted structural and thermodynamic properties of four uranium compounds central to the field of nuclear fuels: UC, UN, UO2, and UCl3. We also assess the impact of the exchange-hole dipole moment (XDM) dispersion correction on the computed properties. Overall, the choice of Hubbard U value and inclusion of a dispersion correction cause larger variations in the computed geometric properties than result from metastable states. The weak dependence of structure optimization on metastable states should simplify future high-throughput calculations on actinides. Conversely, addition of the dispersion correction is found to offset the repulsion introduced by the Hubbard U term and provides greatly improved agreement with experiment for both cell volumes and heats of formation. The XDM dispersion correction is largely invariant to the chosen U value, making it a robust dispersion correction for actinide systems.

Computational workflows for perovskites: case study for lanthanide manganites

“…One remedy may be the use of a dispersion-corrected GGA+ U , such as PBE+ U +XDM, which has been reported to improve upon PBE+ U for cell volumes and formation enthalpies of uranium compounds. 67 To conclude, in order to obtain agreement with the experimental data, it is crucial to allow the defective supercells to relax from the parent stoichiometric geometry. Quantitiative agreement with experiment is in this case obtained with HCTH120.…”

Modeling Metallic Halide Local Structures in Salt Melts Using a Genetic Algorithm

“…This functional was chosen because of its proven ability to accurately model uranium compounds. , “Cold” smearing was used with a parameter of 0.02 eV and a wave-function cutoff of 600 eV. van der Waals interactions were incorporated into the calculations through Grimme’s DFT-D3 correction, a requirement to accurately calculate enthalpies and structures for uranium chlorides. , Convergence criteria for the total energy was set to 1 × 10 –4 eV, and forces were set to 1 × 10 –4 eV/Å. Three total calculations were carried out with successively higher k -point meshes.…”

Computational workflows for novel materials: Case study for lanthanide manganese perovskites

“…One remedy may be the use of the dispersion-corrected GGA+U , such as PBE+U +XDM, which has been reported to improve upon PBE+U for cell volumes and formation enthalpies of uranium compounds. [66] To conclude, in order to obtain agreement with the experimental data, it is crucial to allow the defective supercells to relax from the parent stoichiometric geometry. A partial, quantitiative agreement with experiment is obtained with HCTH120; however, it might be the case of "the right answer for the wrong reasons", as the qualitative behaviour of the system is not captured.…”