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arXiv · 2107.00103

Limitations of coupled cluster approximations for highlyaccurate investigations of Rb$_2^+$

Jan Schnabel·Lan Cheng·Andreas Köhn
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Abstract

We reveal limitations of several standard coupled-cluster (CC) methods with perturbation-theorybased noniterative or approximate iterative treatments of triple excitations when applied to thedetermination of highly accurate potential energy curves (PECs) of ionic dimers, such as the X $^2Σ^+_g$ electronic ground state of Rb$_2^+$. Such computations are of current interest for the understanding ofion-atom interactions in the ultracold regime. We demonstrate that these coupled-cluster methodslead to an unphysical long-range barrier for the Rb$_2^+$ system. The barrier is small but clearly spoilsthe entire long-range behavior of the PEC. The effect is also found for other X$_2^+$ systems, like X = Li, Na, and K. Calculations using a new flexible framework for obtaining leading perturbativetriples corrections derived using an analytic CC singles and doubles (CCSD) energy derivativeformulation demonstrate that the origin of this problem lies in the use of $T_3$ amplitudes obtained from approximate CC singles, doubles and triples (CCSDT) amplitude equations. It is shown thatthe unphysical barrier is related to a symmetry instability of the underlying Hartree-Fock mean-fieldsolution. Physically meaningful perturbative corrections in the long-range tail of the PEC mayinstead be obtained using symmetry-broken reference determinants.

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Jan Schnabel, Lan Cheng, Andreas Köhn. 2021-06-30. Limitations of coupled cluster approximations for highlyaccurate investigations of Rb$_2^+$. https://doi.org/10.1063/5.0062098

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