Double electron-attachment equation-of-motion coupled-cluster methods with up to 4-particle–2-hole excitations: improved implementation and application to singlet–triplet gaps in ortho-, meta-, and para-benzyne isomers

Abstract

Efficient new codes enabling double electron-attachment equation-of-motion coupled-cluster (DEA-EOMCC) calculations with up to 4-particle–2-hole (4p-2h) excitations, treated with active orbitals, have been developed. They can be applied to medium size diradicals using larger basis sets and modest computational resources. The new codes have been used to determine the singlet–triplet (S–T) gaps characterising the ortho-, meta-, and para-benzyne isomers using the correlation consistent cc-pVxZ basis sets with $x=\mathrm{D}$, T, and Q and complete-basis-set (CBS) extrapolations. Our best estimates of the S–T gaps in the ortho-, meta-, and para-benzynes obtained in the DEA-EOMCC calculations with an active-space treatment of 4p-2h terms, extrapolated to the CBS limit and corrected for vibrations, are 37.4, 20.7, and 4.5 kcal/mol, respectively, in excellent agreement with experiment.

GRAPHICAL ABSTRACT

Keywords:

• Equation-of-motion coupled-cluster theory
• double electron attachment
• active-space methods
• diradicals
• benzynes

Acknowledgments

We dedicate this paper to Professor John F. Stanton on the occasion of his 60th birthday. One of us (P.P.) thanks Professors Rodney J. Bartlett, Lan Cheng, Devin A. Matthews, and Péter G. Szalay and Dr. Ajith Perera for inviting him to contribute to the Special Issue of Molecular Physics in honour of Professor John F. Stanton.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This work has been supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (Grant No. DE-FG02-01ER15228).