# Chemically Accurate Excitation Energies With Small Basis Sets

## TL;DR

This paper demonstrates that combining extrapolated sCI energies via CIPSI with a short-range density functional correction yields chemically accurate excitation energies using small basis sets, improving computational efficiency.

## Contribution

The study introduces a novel approach that integrates extrapolated sCI energies with a density-functional correction to achieve accurate excitation energies with minimal basis sets.

## Key findings

- Achieved chemically accurate excitation energies with small basis sets.
- Effective for valence, Rydberg, and double excitations in small molecules.
- Less effective for very diffuse excited states due to radial incompleteness.

## Abstract

By combining extrapolated selected configuration interaction (sCI) energies obtained with the CIPSI (Configuration Interaction using a Perturbative Selection made Iteratively) algorithm with the recently proposed short-range density-functional correction for basis-set incompleteness [Giner et al.,J. Chem. Phys. 2018, 149, 194301], we show that one can get chemically accurate vertical and adiabatic excitation energies with, typically, augmented double-$\zeta$ basis sets. We illustrate the present approach on various types of excited states (valence, Rydberg, and double excitations) in several small organic molecules (methylene, water, ammonia, carbon dimer and ethylene). The present study clearly evidences that special care has to be taken with very diffuse excited states where the present correction does not catch the radial incompleteness of the one-electron basis set.