# Electronic Structure of First and Second Row Atoms under Harmonic Confinement

## TL;DR

This study investigates how harmonic confinement affects the electronic structure of first and second row atoms, revealing pressure-induced ionization, state mixing, and changes in electron correlation, with implications for modeling atoms under extreme conditions.

## Contribution

It provides a systematic analysis of atomic electronic structure under harmonic confinement, introducing a method to relate confinement strength to pressure and examining correlation effects across elements.

## Key findings

- Critical ionization pressures follow a periodic trend from Li to Ne.
- High pressure induces mixing of ground and excited states in Li and Be.
- Electron correlation increases with pressure in lighter atoms but decreases in heavier atoms.

## Abstract

Atoms under pressure undergo a series of processes and modification of its electronic structure. Examples are the spontaneous ionization, stabilization of excited-state configurations that result in a level-crossing with the ground state of the free atom, and contraction of atomic shells. In this work, we do a systematic study of the effects of confinement with harmonic potential on the electronic structure of atoms from H to Ne. Dynamic and static correlation is taken into account by performing CCSD and CASSCF calculations, respectively. Because the strength of harmonic confinement cannot be translated into pressure, we envisioned a "calibration" method to transform confinement into pressure. We focused on the effect of confinement on: i) changes of electron distribution and localization within the $K$ and $L$ atomic shells, ii) confinement-induced ionization pressure, iii) level crossing of electronic states, and iv) the electron correlation energy. We found that contraction of valence and core shells are not negligible and that the use of standard pseudopotentials might be not adequate to study solids under extreme pressures. The critical pressure at which and atom ionizes follows a periodic trend, and it ranges from $28$ GPa for Li to $10.8$ TPa for Ne. In Li and a Be, pressure induces mixing of the ground state configuration with excited states. At high pressure, the ground state of Li and Be becomes a doublet and a triplet with configurations $1s^22p$ and $1s^22s2p$ respectively. The potential consequences of these changes of configuration on the chemistry of Be are discussed. Finally, the changes in the amount of electron correlation are characterized and analyzed in terms of the RPA approximation. For atoms with fewer electrons in the valence shell correlation increases, but for atoms with more electron, the increasing of kinetic energy dominates over electron correlation.