In a similar way, we could enumerate the number of states for a two-dimensional system consisting of two spins, i.e., a \( 2\times 2 \) Ising model on a square lattice with {\em periodic boundary conditions}. In this case we have a total of \( 2^4=16 \) states. Some examples of configurations with their respective energies are listed here $$ \begin{equation*} E=-8J\hspace{1cm}\begin{array}{cc}\uparrow & \uparrow \\ \uparrow & \uparrow\end{array} \hspace{0.5cm} E=0\hspace{1cm}\begin{array}{cc}\uparrow & \uparrow \\ \uparrow & \downarrow\end{array} \hspace{0.5cm} E=0\hspace{1cm}\begin{array}{cc}\downarrow & \downarrow \\ \uparrow & \downarrow\end{array} \hspace{0.5cm} E=-8J\hspace{1cm}\begin{array}{cc}\downarrow & \downarrow \\ \downarrow & \downarrow\end{array} \end{equation*} $$