A Mountaineering Strategy to Excited States: Revising Reference Values with EOM-CC4

TL;DR

This paper demonstrates that CC4 is a highly accurate and computationally efficient method for calculating excited states in small organic molecules, offering improvements over previous methods and refining reference values.

Contribution

The study introduces CC4 as a reliable approximation to CCSDTQ for excited states and evaluates basis set correction schemes, enhancing computational strategies for excitation energy calculations.

Findings

CC4 closely approximates CCSDTQ with minimal deviation (~0.003 eV).

Basis set correction schemes significantly improve excitation energy estimates.

CC4 refines theoretical best estimates in the QUEST database.

Abstract

In the framework of the computational determination of highly-accurate vertical excitation energies in small organic compounds, we explore the possibilities offered by the equation-of-motion formalism relying on the approximate fourth-order coupled-cluster (CC) method, CC4. We demonstrate, using an extended set of more than 200 reference values based on CC including up to quadruples excitations (CCSDTQ), that CC4 is an excellent approximation to CCSDTQ for excited states with a dominant contribution from single excitations with an average deviation as small as $0.003$ eV. We next assess the accuracy of several additive basis set correction schemes, in which vertical excitation energies obtained with a compact basis set and a high-order CC method are corrected with lower-order CC calculations performed in larger basis sets. Such strategies are found to be overall very beneficial, though their accuracy depend significantly on the actual scheme. Finally, CC4 is employed to improve several theoretical best estimates of the QUEST database for molecules containing between four and six (non-hydrogen) atoms, for which previous estimates were computed at the CCSDT level.