The Quest For Highly Accurate Excitation Energies: A Computational Perspective

TL;DR

This paper reviews the evolution of computational methods for accurately calculating excitation energies in molecules, highlighting recent advances with selected configuration interaction methods and benchmarking strategies.

Contribution

It provides a comprehensive overview of the development of high-accuracy computational techniques and benchmark sets for excitation energies, emphasizing recent methodological progress.

Findings

Selected configuration interaction methods yield highly accurate excitation energies for small and medium molecules.

Benchmark sets enable fair assessment of lighter computational methods.

High-level methods have achieved chemically accurate vertical transition energies.

Abstract

We provide an overview of the successive steps that made possible to obtain increasingly accurate excitation energies with computational chemistry tools, eventually leading to chemically accurate vertical transition energies for small- and medium-size molecules. First, we describe the evolution of \textit{ab initio} methods employed to define benchmark values, with originally Roos' CASPT2 method, then the CC3 method as in the renowned Thiel set, and more recently the resurgence of selected configuration interaction methods. The latter method has been able to deliver consistently, for both single and double excitations, highly accurate excitation energies for small molecules, as well as medium-size molecules with compact basis sets. Second, we describe how these high-level methods and the creation of representative benchmark sets of excitation energies have allowed to assess fairly and accurately the performance of computationally lighter methods. We conclude by discussing the future theoretical and technological developments in the field.