# A numerical study of the extended Kohn-Sham ground states of atoms

## TL;DR

This paper numerically investigates the extended Kohn-Sham model for atoms, computing energy levels and responses to electric fields, and compares results with theoretical assumptions and perturbation theory.

## Contribution

It provides a detailed numerical analysis of the extended Kohn-Sham model for atoms, including discretization, algorithm development, and validation against theoretical predictions.

## Key findings

- Computed energy levels for atoms in the first four rows of the periodic table.
- Observed accidental degeneracies between atomic shells.
- Validated density response to electric fields against perturbation theory.

## Abstract

In this article, we consider the extended Kohn-Sham model for atoms subjected to cylindrically-symmetric external potentials. The variational approximation of the model and the construction of appropriate discretization spaces are detailed together with the algorithm to solve the discretized Kohn-Sham equations used in our code. Using this code, we compute the occupied and unoccupied energy levels of all the atoms of the first four rows of the periodic table for the reduced Hartree-Fock (rHF) and the extended Kohn-Sham X$\alpha$ models. These results allow us to test numerically the assumptions on the negative spectra of atomic rHF and Kohn-Sham Hamiltonians used in our previous theoretical works on density functional perturbation theory and pseudopotentials. Interestingly, we observe accidental degeneracies between s and d shells or between p and d shells at the Fermi level of some atoms. We also consider the case of an atom subjected to a uniform electric-field. For various magnitudes of the electric field, we compute the response of the density of the carbon atom confined in a large ball with Dirichlet boundary conditions, and we check that, in the limit of small electric fields, the results agree with the ones obtained with first-order density functional perturbation theory.