Update src/7_tight_binding_model_sol.md

src/7_tight_binding_model_sol.md

@@ -77,3 +77,10 @@ For the heat capacity we have: $$C = \frac{\partial U}{\partial T} = \int g(\ome
 
 The Schrödinger equation is given as: $|\Psi\rangle = \sum_n \phi_n |n\rangle$ such that we find $$ E\phi_n = E_0\phi_n - t\phi_{n-1} - t\phi_{n+1} - t'\phi_{n-2} - t'\phi_{n+2}$$.
 
+### Subquestion 2
+
+Solving the Schrödinger equation yields dispersion: $$E(k) = E_0 -t\cos(ka) -t'\cos(2ka)$$
+
+### Subquestion 3
+
+