= Simple-cubic nearest-neighbour tight-binding band
{title2=$E(\mathbf k)=E_0+2t\sum_{j=1}^3\cos(k_ja),\quad W=12|t|$}
For one orthonormal localized orbital per simple-cubic site and real equal nearest-neighbour <hopping integral> $t$, the six neighbours give the displayed <energy band>. Each cosine ranges from minus one to one, so the positive bandwidth is $12|t|$, independently of the sign convention for $t$. Near the zone centre, $E=E_0+6t-ta^2k^2+O(k^4)$ and the <band effective mass> is $-\hbar^2/(2ta^2)$. It is positive there for $t<0$; for $t>0$ the zone centre is a maximum instead.
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