fixed mixing rule tutorial 4

Author: Simon Gravelle

lammps-tutorials.tex

@@ -2420,8 +2420,8 @@ \subsubsection{System preparation}
 pairwise interactions were only assigned between atoms of identical
 types.  By default, LAMMPS calculates the pair coefficients for the
 interactions between atoms of different types (i and j) by using
-geometric average: $\epsilon_{ij} = (\epsilon_{ii} + \epsilon_{jj})/2$,
-$\sigma_{ij} = (\sigma_{ii} + \sigma_{jj})/2$.  However, if the default
+geometric average: $\epsilon_{ij} = \sqrt{\epsilon_{ii} \epsilon_{jj}}$,
+$\sigma_{ij} = \sqrt{\sigma_{ii} \sigma_{jj}}$.  However, if the default
 value of $5.941\,\text{kcal/mol}$ was used for $\epsilon_\text{1-5}$,
 the solid walls would be extremely hydrophilic, causing the water
 molecules to form dense layers.  As a comparison, the water-water energy