The dashed line is the classical value, $k_{\rm B}$.
Shaded area $=\frac{1}{2}\hbar\omega_0$, the zero point energy that cannot be removed through cooling.
This is for just one atom. In order to obtain the heat capacity of a full material, we would have to multiply $C$ (or $\langle E \rangle$) by $3N$, _i.e._ the number of harmonic oscillators according to Einstein model.
This is for just one atom. In order to obtain the heat capacity of a full material, we would have to multiply $C$ (or $\langle E \rangle$) by $3N$, _i.e._ the number of harmonic oscillators according to Einstein model. The plot below shows a fit of the Einstein model to the experimental data for the heat capacity of diamond.