Strongly separated pairs of core electrons in computed ground states of small molecules

TL;DR

This study uses full configuration interaction calculations to demonstrate that core electrons in small molecules form strongly separated pairs with nontrivial structures, verified through direct wavefunction analysis.

Contribution

It introduces a direct, conclusive method to identify separated electron pairs in molecular ground states without fitting models, based on wavefunction purity.

Findings

Core electrons form separated pairs in studied molecules.

The core pairs have complex structures beyond single orbitals.

The method confirms the presence of quasi-separated pairs directly from wavefunctions.

Abstract

We have performed full configuration interaction computations of the ground states of the molecules Be, BeH_2, Li, LiH, B, and BH and verified that the core electrons constitute "separated electron pairs." These separated pairs of core electrons have nontrivial structure; the core pair does not simply occupy a single spatial orbital. Our method of establishing the presence of separated electron pairs is direct and conclusive. We do not fit a separated pair model; we work with the wavefunctions of interest directly. To establish that a given group of spin-orbitals contains a quasi-separated pair, we verify by direct computation that the quantum state of the electrons that occupy those spin-orbitals is nearly a pure 2-electron state.