  Ordinary band structure calculations of quantum materials often incorrectly
predicted metallic, instead of insulating electronic structure, motivating
Mott-Hubbard strong electron correlation as a gapping mechanism. More recently,
allowing the formation of local structural symmetry breaking motifs in
otherwise ordinary band theory was shown to lower the total energy while
predicting insulating gaps when they are observed. An important counter example
was recently pointed out whereby the flat band formed in Nb3Cl8 by symmetry
breaking is by itself partially occupied, thus failing to account for the
observed insulating state. It is shown here that a generalized symmetry
breaking involving the cooperative structural and magnetic effects produces in
mean-field-like density functional theory an energy lowering insulating phase.
