A Mountaineering Strategy to Excited States: Highly-Accurate Energies and Benchmarks for Bicyclic Systems

TL;DR

This paper provides highly accurate vertical excitation energies for 10 bicyclic molecules using coupled-cluster methods and benchmarks various wave function methods against these references to assess their accuracy across different system sizes.

Contribution

It offers a comprehensive set of benchmark excitation energies for large bicyclic molecules and evaluates the performance of multiple computational methods in this context.

Findings

CIS(D) and CC3 methods maintain accuracy regardless of system size.

EOM-MP2 and CCSD performance deteriorates with larger molecules.

ADC(2) and CC2 methods improve accuracy for bigger systems.

Abstract

Pursuing our efforts to define highly-accurate estimates of the relative energies of excited states in organic molecules, we investigate, with coupled-cluster methods including iterative triples (CC3 and CCSDT), the vertical excitation energies of 10 bicyclic molecules (azulene, benzoxadiazole, benzothiadiazole, diketopyrrolopyrrole, fuofuran, phthalazine, pyrrolopyrrole, quinoxaline, tetrathiafulvalene, and thienothiophene). In total, we provide \emph{aug}-cc-pVTZ reference vertical excitation energies for 91 excited states of these relatively large systems. We use these reference values to benchmark various wave function methods, i.e., CIS(D), EOM-MP2, CC2, CCSD, STEOM-CCSD, CCSD(T)(a)*, CCSDR(3), CCSDT-3, ADC(2), ADC(2.5), ADC(3), as well as some spin-scaled variants of both CC2 and ADC(2). These results are compared to those obtained previously on smaller molecules. It turns out that while the accuracy of some methods is almost unaffected by system size, e.g., CIS(D) and CC3, the performance of others can significantly deteriorate as the systems grow, e.g., EOM-MP2 and CCSD, whereas others, e.g., ADC(2) and CC2, become more accurate for larger derivatives.