Brueckner coupled-cluster effect

Since Brueckner orbitals are defined as those for which all the single excitation amplitudes in a particular correlation method (usually coupled cluster, see below) exactly vanish, which means that T = 0, electron correlation calculations based on Brueckner orbitals generally are much less likely to be plagued by false symmetry breaking problems. And this has indeed been the observation in such systems as NO3 and O4, which are notorious for symmetry breaking effects. 

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