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Performance of valence‐universal multireference coupled‐cluster theory for quasi‐degenerate states: The H 8 and DZP H 4 models
Author(s) -
Jankowski K.,
Grabowski I.
Publication year - 1995
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560550215
Subject(s) - coupled cluster , valence (chemistry) , degenerate energy levels , basis set , chemistry , degeneracy (biology) , atomic orbital , computational chemistry , atomic physics , physics , quantum mechanics , molecule , density functional theory , bioinformatics , biology , electron
Valence‐universal coupled‐cluster ( VU – CC ) methods corresponding to the approximations S ≃ S 1 + S 2 ( VU – CCSD ) and S ≃ S 1 + S 2 + S 3 ( VU – CCSDT ) for the cluster operator S have been applied to two model systems for which the degree of quasi‐degeneracy of the two lowest, totally symmetric, singlet states may be varied over a wide range by changing their geometry. The performance of the VU – CC methods is investigated with the aim of verifying the findings of recent extensive reliability studies of these methods [Jankowski et al., J. Chem. Phys. 97 , 7600 (1992); Ibid . 101 , 3085 (1994)] based on calculations for the less complex minimum basis set H 4 model. The present results confirm these findings. In particular, the VU – CCSD energies are less accurate than those obtained with other single‐ and multireference methods including up two‐body amplitudes. It is also confirmed that the impact of the S 3 clusters is significant and the accuracy of description is rather insensitive to the kind of molecular orbitals employed. © 1995 John Wiley & Sons, Inc.