  As shown in former papers, the nonadiabatic Heisenberg model presents a novel
mechanism of Cooper pair formation generated by the strongly correlated
atomic-like motion of the electrons in narrow, roughly half-filled
"superconducting bands". These are energy bands represented by optimally
localized spin-dependent Wannier functions adapted to the symmetry of the
material under consideration. The formation of Cooper pairs is not the result
of an attractive electron-electron interaction but can be described in terms of
quantum mechanical constraining forces constraining the electrons to form
Cooper pairs. There is theoretical and experimental evidence that only this
nonadiabatic mechanism operating in superconducting bands may produce
eigenstates in which the electrons form Cooper pairs. These constraining forces
stabilize the Cooper pairs in any superconductor, whether conventional or
unconventional. Here we report evidence that also the experimentally found
superconducting state in bismuth at ambient as well as at high pressure is
connected with a narrow, roughly half-filled superconducting band in the
respective band structure. This observation corroborates once more the
significance of constraining forces in the theory of superconductivity.
