${\bf a}={\bf u}({\bf R})-{\bf u}({\bf R'})$
and only retain terms quadratic in $u$.
\item Use the evaluated potential to calculate the energy density
- (do not forget the kinetic contribution to energy) and
+ (do not forget the kinetic energy contribution) and
the specific heat $c_{\text{V}}$.
{\bf Hint:}
Use the following change of variables
\section{Specific heat in the quantum theory of the harmonic crystal -\\
- Models of Debye and Einstein}
+ The Debye model}
\begin{enumerate}
\item