Document Type

Article

Publication Date

11-2017

Abstract

Coupled cluster (CC) theory is widely accepted as the most accurate and generally applicable approach in quantum chemistry. CC calculations are usually performed with single Slater-determinant references, e.g., canonical Hartree-Fock (HF) wavefunctions, though any single determinant can be used. This is an attractive feature because typical CC calculations are straightforward to apply, as there is no potentially ambiguous user input required. On the other hand, there can be concern that CC approximations give unreliable results when the reference determinant provides a poor description of the system of interest, i.e., when the HF or any other single determinant ground state has a relatively low weight in the full CI expansion. However, in many cases, the reported “failures” of CC can be attributed to an unfortunate choice of reference determinant, rather than intrinsic shortcomings of CC itself. This is connected to well-known effects like spin-contamination, wavefunction instability, and symmetry-breaking. In this contribution, a particularly difficult singlet/triplet splitting problem in two phenyldinitrene molecules is investigated, where CC with singles, doubles and perturbative triples [CCSD(T)] was reported to give poor results. This is analyzed by using different reference determinants for CCSD(T), as well as performing higher level CCSDT-3 and CCSDT calculations. We show that doubly electron attached and doubly ionized equation-of-motion (DEA/DIP-EOM) approaches are powerful alternatives for treating such systems. These are operationally single-determinant methods that adequately take the multi-reference nature of these molecules into account. Our results indicate that CC remains a powerful tool for describing systems with both static correlation and dynamic correlation, when pitfalls associated with the choice of the reference determinant are avoided.

Comments

© 2017 Authors, published under an exclusive license with American Institute of Physics.

AFIT Scholar, as the repository of the Air Force Institute of Technology, furnishes the published Version of Record for this article in accordance with the sharing policy of the publisher, AIP Publishing. A 12-month embargo was observed.

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing.

This article appeared in The Journal of Chemical Physics, 147, 184101 (2017) and may be found at https://doi.org/10.1063/1.5003128.

Funding notes: This work has been supported by the U. S. Army Research Office, under Grant No. W911NF-16-1-0260, and by the Alexander-von-Humboldt Foundation (via a Feodor Lynen Fellowship for J.T.M. and a Humboldt Research Award for R.J.B.). J.J.L. was supported in part by an appointment to the Faculty Research Participation Program at the U.S. Air Force Institute of Technology (AFIT), administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and AFIT.

DOI

10.1063/1.5003128

Source Publication

The Journal of Chemical Physics

Share

COinS