Since most materials condense into almost perfect periodic arrays the periodicity should also hold for the potential style.
Within this tutorial even the periodic potential is simplified.
-Consider a single particle (mass $m$) enclosed in a box (side length $L=\mathcal{V}^{1/3}$) where the potential is constant ($V_0$) inside the box and infinite at the surface.
+Consider a single particle (mass $m$) enclosed in a box (side length $L=\mathcal{V}^{1/3}$) where the potential is zero inside the box and infinite at the surface.
\begin{enumerate}
\item Write down the Schr"odinger equation and boundary conditions