Breaking phenomena

Another aspect of wave function instability concerns symmetry breaking, i.e. the wave function has a lower symmetry than the nuclear framework. It occurs for example for the allyl radical with an ROHF type wave function. The nuclear geometry has C21, symmetry, but the Cay symmetric wave function corresponds to a (first-order) saddle point. The lowest energy ROHF solution has only Cj symmetry, and corresponds to a localized double bond and a localized electron (radical). Relaxing the double occupancy constraint, and allowing the wave function to become UHF, re-establish the correct Cay symmetry. Such symmetry breaking phenomena usually indicate that the type of wave function used is not flexible enough for even a qualitatively correct description. 

The fact that states with definite parities are almost never observed experimentally cannot be explained alone by the persistence of mixed-parity states. Obviously, symmetry-breaking phenomena are in operation whether or not external perturbations are present. For species in condensed media, symmetry breaking is brought about by intermolecular interactions. In the extreme case of solids, these interactions are so strong that it is proper... 

The Lithium Superoxide Radical Symmetry Breaking Phenomena and Potential Energy Surfaces. 

In order to characterize the bending or breaking phenomena we have defined an apparent elastic modulus (Ga) as follows ... 

The transgranulate break phenomena of crack are not observed in the PLS specimen (Fig. 3b). But it does not mean that there is no transgranular fracture inside the specimen because the superficial crystal grain environment is completely different from inside. 

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