# Targeting excited states in all-trans polyenes with electron-pair states

## TL;DR

This paper extends the pCCD method to model excited states in strongly-correlated systems, offering a simpler and cost-effective alternative to traditional methods, with promising results on polyenes and uranyl cation.

## Contribution

The authors develop an EOM-pCCD method with single excitations to efficiently target singly excited states in strongly-correlated molecules.

## Key findings

- EOM-pCCD with single excitations accurately describes excited states.
- The method performs well on polyenes and uranyl cation.
- It offers a computationally cheaper alternative to EOM-CC methods.

## Abstract

Wavefunctions restricted to electron pair states are promising models for strongly-correlated systems. Specifically, the pair Coupled Cluster Doubles (pCCD) ansatz allows us to accurately describe bond dissociation processes and heavy-element containing compounds with multiple quasi-degenerate single-particle states. Here, we extend the pCCD method to model excited states using the equation of motion (EOM) formalism. As the cluster operator of pCCD is restricted to electron-pair excitations, EOM-pCCD allows us to target excited electron-pair states only. To model singly excited states within EOM-pCCD, we modify the configuration interaction ansatz of EOM-pCCD to contain also single excitations. Our proposed model represents a simple and cost-effective alternative to conventional EOM-CC methods to study singly excited electronic states. The performance of the excited state models is assessed against the lowest-lying excited states of the uranyl cation and the two lowest-lying excited states of all-trans polyenes. Our numerical results suggest that EOM-pCCD including single excitations is a good starting point to target singly excited states.