small grammar tweak

Author: simongravelle

lammps-tutorials.tex

@@ -712,8 +712,9 @@ \subsubsection{My first input}
 \end{figure}
 
 The potential energy, $U$, decreases from a positive value to a negative value
-(Figs.~\ref{fig:chart-log} and~\ref{fig:evolution-energy}\,a).  Note that $E = U$ 
-during energy minization, as the kinetic energy $K = 0$.  The
+(Figs.~\ref{fig:chart-log} and~\ref{fig:evolution-energy}\,a).  Note that
+during energy minization, the potential energy equals the total energy
+of the system, $E = U$, since the kinetic energy, $K$, is zero.  The
 initially positive potential energy is expected, as the atoms are
 created at random positions within the simulation box, with some in very
 close proximity to each other.  This proximity results in a large