Density-functional theory (DFT) is the workhorse method to model nuclear material properties. The results of these calculations can be used as inputs for multi-scale, multi-physics codes, such as BISON, to model fuel performance in nuclear reactors. It is therefore imperative that calculated results are at least qualitatively correct. However, it has been shown that the ground state energy of uranium compounds greatly depends on the f-orbital occupations. The semi-local nature of DFT functionals does not necessarily place electrons in the lowest energy orbital configuration. Non-physical fractional occupations can also occur. It is possible that orbital occupations could affect the calculation of physical properties, such as lattice constant and bond lengths. This talk presents a systematic study of how orbital configurations affect calculated physical and energetic properties for UC, UCl3, UN and UO2. The effect of how Hubbard U and dispersion corrections affect orbital occupancy is also discussed.

Dependence on orbital occupations for density-functional theory (DFT) calculations of uranium compounds

ACS Fall 2020 Virtual Meeting

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