functions

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<!-- MATH
 \begin{eqnarray*}
w_A(r) & = & \prod_{B\neq A}\frac{1}{2} \left[1 \ - \
erf(\mu^\prime_{AB})\right] \\
 \mu^\prime_{AB} & = & \frac{1}{\alpha} \ \frac{\mu_{AB}}{(1-\mu_{AB}^2)^n}\\
 \mu_{AB} & = & \frac{{\mathbf r}_A - {\mathbf r}_B}
{\left|{\mathbf r}_A - {\mathbf r}_B \right|}
\end{eqnarray*}
 -->
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w_A(r) &amp; = &amp; \prod_{B\neq A}\frac{1}{2} \left[1 \ - \
erf(\mu...