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Updated: Mar 10, 2026

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
Published on: April 8, 2020
Accuracy of Coupled Cluster Excitation Energies in Diffuse Basis Sets
Dániel Kánnár1, Attila Tajti1, Péter G Szalay1
1Laboratory of Theoretical Chemistry, Institute of Chemistry, Eötvös Loránd University , P.O. Box 32, H-1518, Budapest 112, Hungary.
Abstract:
We present a comprehensive statistical analysis on the accuracy of various excited state Coupled Cluster methods, accentuating the effect of diffuse basis sets on vertical excitation energies of valence and Rydberg-type states. Many popular approximate doubles and triples methods are benchmarked with basis sets up to aug-cc-pVTZ, with high level EOM-CCSDT results used as reference. The results reveal a serious deficiency of CC2 linear response and CIS(D) techniques in the description of Rydberg states, a feature not shown by the EOM-CCSD(2) and EOM-CCSD variants. The CC3 theory proves to be an accurate choice among the iterative approximate triples methods, while the novel perturbation-based CCSD(T)(a)* variant turns out to be the best way to include the effect of triple excitations in a noniterative way.
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