Application of the coupled-cluster CC(P;Q) approaches to the magnesium dimer

ABSTRACT

Several coupled-cluster approaches with an approximate treatment of connected triply or quadruply excited clusters, including the CCSD(T), ${\mathrm{CCSD}\left(2\right)}_{\mathrm{T}}$, and CR-CC(2,3) corrections to CCSD, the active-space CCSDt and CCSDTq methods, and the CC(t;3) and CC(q;4) corrections to CCSDt and CCSDTq derived using the CC(P;Q) framework, and their CCSDT and CCSDTQ parents are applied to the ground-state potential curve and vibrational term values of the magnesium dimer. The correlation-consistent aug-cc-pV(n+d)Z and aug-cc-pwCVnZ ($n=\mathrm{T}$ and Q) basis sets are used. Among the noniterative triples corrections to CCSD, the CR-CC(2,3) approach performs the best, but, in analogy to the previously studied beryllium dimer [I. Magoulas et al., J. Phys. Chem. A 122, 1350 (2018)], the CC(t;3) and CC(q;4) methods outperform other employed approaches in reproducing the CCSDT and CCSDTQ data. Composite calculations combining the nearly all-electron CCSDT and valence CC(q;4) and CCSDTQ computations reproduce the experimentally resolved part of the vibrational spectrum of ${\mathrm{Mg}}_2$ to within ∼1 ${\mathrm{cm}}^{-1}$ when the vibrational spacings are examined, while predicting five extra vibrational states near the dissociation threshold, which have not been experimentally resolved. The corresponding binding energies and equilibrium bond lengths are within a few ${\mathrm{cm}}^{-1}$ and 0.01 Å from their experimentally derived values.

GRAPHICAL ABSTRACT

KEYWORDS:

• Coupled-cluster theory
• higher-than-two-body clusters
• magnesium dimer
• potential energy curves
• vibrational spectrum

Acknowledgments

We dedicate this article to the memory of the late Professor Dieter Cremer, whose insightful work in many areas of theoretical chemistry, including his seminal contributions to the CC theory, continues to provide us with a lot of inspiration. One of us (P.P.) would like to thank Professor Elfi Kraka for inviting him to submit an article for the Special Memorial Issue of Molecular Physics in honour of Professor Dieter Cremer.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Stephen H. Yuwono http://orcid.org/0000-0002-6604-3543

Ilias Magoulas http://orcid.org/0000-0003-3252-9112

Jun Shen http://orcid.org/0000-0003-1838-3719

Piotr Piecuch http://orcid.org/0000-0002-7207-1815

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).