lammps/doc/Eqs/fix_orient_fcc.tex

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\newcommand{\br}{\mathbf{r}}
\documentclass[12pt]{article}
\begin{document}
\begin{equation}
\xi_{i} = \sum_{j=1}^{12} \left| \br_{j} - \br_{j}^{\rm I} \right|
\end{equation}
\begin{equation}
\xi_{\rm IJ} = \sum_{j=1}^{12} \left| \br_{j}^{\rm J} - \br_{j}^{\rm I} \right|
\end{equation}
\begin{eqnarray}
\xi_{\rm low} &=& {\rm cutlo} \, \xi_{\rm IJ} \\
\xi_{\rm high} &=& {\rm cuthi} \, \xi_{\rm IJ}
\end{eqnarray}
\begin{eqnarray}
\omega_{i} &=& \frac{\pi}{2}
\frac{\xi_{i} - \xi_{\rm low}}{\xi_{\rm high} - \xi_{\rm low}} \\
\mbox{\hspace*{72pt}} \nonumber \\
u_{i} &=& 0 \mbox{\hspace{84pt} for } \xi_{i} < \xi_{\rm low} \nonumber \\
&=& {\rm dE}\,\frac{1 - \cos(2 \omega_{i})}{2}
\mbox{\hspace{10pt} for } \xi_{\rm low} < \xi_{i} < \xi_{\rm high} \\
&=& {\rm dE} \mbox{\hspace{80pt} for } \xi_{\rm high} < \xi_{i}
\nonumber
\end{eqnarray}
\end{document}