# Automated Identification of Relevant Frontier Orbitals for Chemical Compounds and Processes

## TL;DR

This paper reviews how entanglement entropy of frontier orbitals relates to static electron correlation and introduces an automated, entanglement-based method for selecting active orbital spaces in quantum-chemical calculations, improving consistency across states and reactions.

## Contribution

It presents an automated scheme for selecting active orbital spaces based on entanglement measures, extending its application to multiple excited states and reaction pathways.

## Key findings

- Automated active space selection based on entanglement entropy is effective.
- The method provides consistent active spaces across different electronic states.
- Application demonstrated along reaction coordinates.

## Abstract

Quantum-chemical multi-configurational methods are required for a proper description of static electron correlation, a phenomenon inherent to the electronic structure of molecules with multiple (near-)degenerate frontier orbitals. Here, we review how a property of these frontier orbitals, namely the entanglement entropy is related to static electron correlation. A subset of orbitals, the so-called active orbital space, is an essential ingredient for all multi-configurational methods. We proposed an automated selection of this active orbital space, that would otherwise be a tedious and error prone manual procedure, based on entanglement measures. Here, we extend this scheme to demonstrate its capability for the selection of consistent active spaces for several excited states and along reaction coordinates.