# Targeting doubly-excited states with Equation of Motion Coupled Cluster theory restricted to double excitations

## TL;DR

This paper introduces a new EOM coupled cluster method tailored for doubly-excited states, combining pCCD and LCCSD to improve accuracy in challenging electronic excitations.

## Contribution

The authors develop EOM-pCCD-LCCSD, a simplified yet accurate approach for describing doubly-excited states, outperforming traditional methods in specific molecular systems.

## Key findings

- Accurately predicts excitation energies in polyenes.
- Outperforms conventional EOM-CCSD in state ordering.
- Closest to experimental results among tested methods.

## Abstract

The accurate description of doubly-excited states using conventional electronic structure methods is remarkably challenging, primarily because such excited states require the inclusion of doubly or higher excited configurations or the application of multi-reference methods. We present a new approach to target electronically excited states that feature a double-electron transfer. Our method uses the equation of motion (EOM) formalism with a pair coupled cluster doubles (pCCD) reference function, where dynamical correlation is accounted for by a linearized coupled cluster correction with singles and doubles (LCCSD). Specifically, our proposed EOM-pCCD-LCCSD model represents a simplification of the conventional EOM-CCSD approach, where the electron-pair amplitudes of CCSD are tailored by pCCD. The performance of EOM-pCCD-LCCSD is assessed for the lowest-lying excited states in CH$^+$ and all-trans polyenes. In contrast to conventional EOM-CC methods with at most double excitations, EOM-pCCD-LCCSD predicts the right order of states in polyenes with excitation energies closest to experiment, outperforming even highly-accurate methods such as the density matrix renormalization group algorithm.