Update src/7_tight_binding_model_sol.md

src/7_tight_binding_model_sol.md

@@ -72,3 +72,5 @@ For the energy we have: $$U = \int \hbar \omega g(\omega) (n(\omega,T) + \frac{1
 
 For the heat capacity we have: $$C = \frac{\partial U}{\partial T} = \int \hbar\omega g(\omega) \frac{\partial n(\omega,T)}{\partial T}d\omega$$
 
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