Symmetry-breaking and near-symmetry-breaking in three-electron-bonded radical cations

The manifestations of the symmetry-breaking artifact in three-electron-bond ed systems have been investigated at several computational levels including second-order Moller-Plesset perturbation theory (MP2), coupled cluster (CC ), and Bruckner-coupled cluster (B-CC) theories. The model systems, [H(n)X4 XH(n)]+(X=Ne, F, O, N, Ar, Cl, S, P; n=0-3) cover all types of three-electr on 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 Moller-Ples set levels are determined for each model system. Their magnitude are found to obey regular tendencies which are related to the compactness of the orbi tals 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 equilibr ium bonding distance between the XHn fragments. The symmetry-breaking artif act results in severe discontinuities in the dissociation curves at the MP2 level. The CC level pushes away the occurrence of the artifact to larger d istances 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 substitue nt effects, to the extent that it becomes shorter than the equilibrium bond ing distance like in the Me4O2+ and Me2F2+ cation radicals that are found t o be symmetry-unstable even in their equilibrium geometries. The artifact c arries over to unsymmetrical systems that display close functional resembla nce to symmetrical systems, leading to convergence difficulties, erroneous geometries, and unphysical localization of the electronic charge. An econom ical alternative to the MP2 method, based on the average quadratic coupled- clusters (AQCC), is proposed for such cases, or in cases some stretched thr ee-electron-bonded systems or full dissociation curves are to be investigat ed. (C) 2001 American Institute of Physics.