Why is the ground state electron configuration for Lithium $1s^22s$ ?

TL;DR

The paper clarifies why Lithium's ground state electron configuration is $1s^22s$, demonstrating that electron-electron interactions, not shielding, determine the energy ordering.

Contribution

It challenges the traditional shielding explanation and shows that electron-electron interaction energies explain the ground state configuration.

Findings

Electron-electron interactions are the key factor in energy ordering.

Shielding arguments are flawed in explaining the ground state.

The $2s$ electron has lower electron-electron repulsion with the core electrons.

Abstract

The electronic ground state for Lithium is $1s^22s$, and not $1s^22p$. The traditional argument for why this is so is based on a screening argument that claims that the $2p$ electron is better shielded by the $1s$ electrons, and therefore higher in energy then the configuration that includes the $2s$ electron. We show that this argument is flawed, and in fact the actual reason for the ordering is because the electron-electron interaction energy is higher for the $2p-1s$ repulsion than it is for the $2s-1s$ repulsion.