${\bf a}={\bf u}({\bf R})-{\bf u}({\bf R'})$
and only retain terms quadratic in $u$.
\item Use the evaluated potential to calculate the energy density
${\bf a}={\bf u}({\bf R})-{\bf u}({\bf R'})$
and only retain terms quadratic in $u$.
\item Use the evaluated potential to calculate the energy density