Near-degeneracy effects structure

One of the most powerful tools presently available for accurate electronic structure calculations is the multiconfiguration reference CI(SD) method. In MR-CI(SD) wavefunctions, all configurations that are singly or doubly excited relative to any of the reference eonfigurations are taken into account, and their coefficients are determined variationally. The reference wavefunctions are usually optimized by the MCSCF method. They should properly describe the dissociation of bonds and near-degeneracy effects. If the reference wavefunction includes the most important double excitations from the... 

A curious effect, prone to appear in near degeneracy situations, is the artifactual symmetry breaking of the electronic wave function [27]. This effect happens when the electronic wave function is unable to reflect the nuclear framework symmetry of the molecule. In principle, an approximate electronic wave function will break symmetry due to the lack of some kind of non-dynamical correlation. A typical example of this case is the allyl radical, which has C2v point group symmetry. If one removes the spatial and spin constraints of its ROHF wave function, a lower energy symmetry broken (Cs) solution is obtained. However, if one performs a simple CASSCF or a SCVB [28] calculation in the valence pi space, the symmetry breaking disappears. On the other hand, from the classical VB point of view, the bonding of the allyl radical is represented as a superposition of two resonant structures. 

In cases of near-degeneracy in D or A manifolds, one component state may couple effectively to the B (hopping integral T in Equation (15)), while the other couples weakly due to the nodal structure. An example is provided in Table 3 for the case of Fc and Fc+ linked by unsaturated Bs of the type APA and VPV, where A, E, and P denote, respectively, an acetyenic, an ethylenic, and a / -phenylenic moiety. In the isolated ferrocene molecule the and "id y orbitals (and... 

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