Past exam of the chemistry A course of the University of Cambridge / 2022 / Paper 2 / A9 / b / iv / Solution

For $0<\gamma <t$, the positive branch ranges from $2\gamma$ to $2t$, and the negative branch from $-2t$ to $-2\gamma$. ThereforeThe smallest separation is between the upper-band minimum and lower-band maximum at the same wavevector $k=\pi /(2a)$:It is a direct band gap at the interior avoided crossing. The zero-mixing limit has zero gap; the width is not $4|\beta |$ because these are the mixed energy-ordered branches, each turning around at the middle of the plotted interval.