The manifestations of the symmetry-breaking artifact in three-electron-bonded systems have been investigated at several computational levels including second-order Møller–Plesset perturbation theory (MP2), coupled cluster (CC), and Bruckner-coupled cluster (B-CC) theories. The model systems, cover all types of three-electron bonds that can possibly take place between atoms of the second and third rows of the Periodic Table. The critical interatomic distance beyond which symmetry breaking begins to take place at the Hartree–Fock and Møller–Plesset levels are determined for each model system. Their magnitude are found to obey regular tendencies which are related to the compactness of the orbitals involved in the three-electron bonds. In all model systems, the onsets of symmetry-breaking at the MP2 level are greater or equal to the equilibrium bonding distance between the fragments. The symmetry-breaking artifact results in severe discontinuities in the dissociation curves at the MP2 level. The CC level pushes away the occurrence of the artifact to larger distances but do not remove the discontinuities. The artifact is practically cured at the B-CC level with perturbative treatment of triple excitations. The onset of symmetry-breaking may in some cases be shortened by substituent effects, to the extent that it becomes shorter than the equilibrium bonding distance like in the and cation radicals that are found to be symmetry-unstable even in their equilibrium geometries. The artifact carries over to unsymmetrical systems that display close functional resemblance to symmetrical systems, leading to convergence difficulties, erroneous geometries, and unphysical localization of the electronic charge. An economical alternative to the MP2 method, based on the average quadratic coupled-clusters (AQCC), is proposed for such cases, or in cases some stretched three-electron-bonded systems or full dissociation curves are to be investigated.
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1 July 2001
Research Article|
July 01 2001
Symmetry-breaking and near-symmetry-breaking in three-electron-bonded radical cations
Benoı̂t Braı̈da;
Benoı̂t Braı̈da
Laboratoire de Chimie-Physique, Groupe de Chimie Théorique, Université de Paris-Sud, 91405 Orsay Cedex, France
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David Lauvergnat;
David Lauvergnat
Laboratoire de Chimie-Physique, Groupe de Chimie Théorique, Université de Paris-Sud, 91405 Orsay Cedex, France
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Philippe C. Hiberty
Philippe C. Hiberty
Laboratoire de Chimie-Physique, Groupe de Chimie Théorique, Université de Paris-Sud, 91405 Orsay Cedex, France
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Benoı̂t Braı̈da
David Lauvergnat
Philippe C. Hiberty
Laboratoire de Chimie-Physique, Groupe de Chimie Théorique, Université de Paris-Sud, 91405 Orsay Cedex, France
J. Chem. Phys. 115, 90–102 (2001)
Article history
Received:
January 31 2001
Accepted:
March 30 2001
Citation
Benoı̂t Braı̈da, David Lauvergnat, Philippe C. Hiberty; Symmetry-breaking and near-symmetry-breaking in three-electron-bonded radical cations. J. Chem. Phys. 1 July 2001; 115 (1): 90–102. https://doi.org/10.1063/1.1373427
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