Analytic gradients for equation-of-motion coupled cluster with single, double, and perturbative triple excitations

TL;DR

This paper derives and implements the first analytic gradients for the EOM-CCSD* method, enhancing the ability to study excited-state potential energy surfaces with high accuracy in complex electronic systems.

Contribution

The paper introduces the first derivation and implementation of analytic gradients for the EOM-CCSD* method, enabling more precise excited-state analyses.

Findings

Successfully derived and implemented EOM-CCSD* analytic gradients.

Demonstrated the method's effectiveness on representative examples.

Enhanced excited-state property calculations with higher accuracy.

Abstract

Understanding the process of molecular photoexcitation is crucial in various fields, including drug development, materials science, photovoltaics, and more. The electronic vertical excitation energy is a critical property, for example in determining the singlet-triplet gap of chromophores. However, a full understanding of excited-state processes requires additional explorations of the excited-state potential energy surface and electronic properties, which is greatly aided by the availability of analytic energy gradients. Owing to its robust high accuracy over a wide range of chemical problems, equation-of-motion coupled-cluster with single and double excitations (EOM-CCSD) is a powerful method for predicting excited state properties, and the implementation of analytic gradients of many EOM-CCSD (excitation energies, ionization potentials, electron attachment energies, etc.) along with numerous successful applications highlights the flexibility of the method. In specific cases where a higher level of accuracy is needed or in more complex electronic structures, the inclusion of triple excitations becomes essential, for example, in the EOM-CCSD* approach of Saeh and Stanton. In this work, we derive and implement for the first time the analytic gradients of EOMEE-CCSD*, which also provides a template for analytic gradients of related excited state methods with perturbative triple excitations. The capabilities of analytic EOMEE-CCSD* gradients are illustrated by several representative examples.