Tailored pseudopotentials for magnesium surface core level shift calculations

In x-ray photoelectron spectroscopy (XPS), identifying the origin of peaks in the spectrum can be guided by theory calculations. With density functional theory (DFT), using pseudopotentials, one can obtain the difference in photoelectron energy for electrons originating from atoms of different environments, for example surface and bulk atoms, and thus model the surface core level shift (SCLS) energies. The focus in this work is to prepare for calculations of magnesium (Mg) <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mn>2</a:mn><a:mi>p</a:mi></a:mrow></a:math> SCLSs in material systems where dispersion interactions play a role, primarily in Mg surface degradation and adsorption of molecules as relevant, for example, in degradable bioimplants. For SCLS DFT calculations in metallic surfaces, the state of the photoelectron must be treated as a core state. In the case of Mg, standard pseudopotentials treat the <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mn>2</b:mn><b:mi>p</b:mi></b:mrow></b:math> state as a valence state, not a core state. We therefore need Mg pseudopotentials with the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mn>2</c:mn><c:mi>p</c:mi></c:mrow></c:math> electrons in the core, leaving only two electrons for the valence region (the <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mn>3</d:mn><d:mi>s</d:mi></d:mrow></d:math> electron); Two-valence electron pseudopotentials are not common, because DFT calculations of Mg-containing materials usually are better or more easily described using 10 valence electrons. In this work, new two-electron Mg pseudopotentials are therefore created for use in dispersion-inclusive DFT calculations. To our knowledge, no such two-electron Mg pseudopotentials exist, proven to work well with the nonlocal, dispersion-inclusive, exchange-correlation functional vdW-DF-cx or similar functionals. We create a number of two-electron Mg pseudopotentials and their <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mn>2</e:mn><e:mi>p</e:mi></e:mrow></e:math>-hole partners, and for four of the most promising we assess their performance in vdW-DF-cx. We provide results for Mg and MgO bulk phases and for the Mg(0001) surface energy, structure, and SCLS, and where possible we compare with results that we obtain by using conventional 10-electron Mg pseudopotentials, all-electron calculations, and with experiments. This work not only reports and tests the specific conditions for creating <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mn>2</f:mn><f:mi>p</f:mi></f:mrow></f:math> Mg results, it will also, we believe, be of help in creating other similar pseudopotentials to aid the analysis of XPS spectra in other materials. Published by the American Physical Society 2024