Symmetry breaking in a nutshell: The odyssey of a pseudo problem in molecular physics. The $\tilde X\,{}^2\Sigma _u^ + $X̃Σu+2 BNB case revisited

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The X̃(2)Σu (+) BNB state considered to be of symmetry broken (SB) character has been studied by high level multireference variational and full configuration interaction methods. We discuss in great detail the roots of the so-called SB problem and we offer an in depth analysis of the unsuspected reasons behind the double minimum topology found in practically all previous theoretical investigations. We argue that the true reason of failure to recover a D∞h equilibrium geometry lies in the lack of the correct permutational symmetry of the wavefunctions employed and is by no means a real effect.