Abstract
An algorithm of unitarily transforming a set of occupied molecular orbitals into a set of localized orbitals (LMOs), which optimally resembles a set of guess LMOs, is applied to investigate degeneracy among LMO sets. It is shown that (1) systems of D4h symmetry (e.g., cyclobutadienedianion) can exhibit continuous degeneracy of LMOs, although this was excluded by a symmetry theorem in the literature; (2) systems of D3h symmetry (e.g., Kekule benzene, borazine) do not exhibit continuous LMO degeneracy, although this was to be expected on the basis of the above-mentioned theorem; and (3) some systems of D6h symmetry (e.g., benzene) exhibit continuous LMO degeneracy while others of the same symmetry (e.g., coronene) do not. For small molecules and clusters, there are rather few cases of discrete degeneracy of LMO sets and even less cases of discrete near degeneracy. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 76: 420–427, 2000
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