\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath}
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\begin{document}
\begin{equation*}
J_\text{ee} = \frac{a_\text{ee} |\mathbf{r}_1 - \mathbf{r}_2|}{1 + b_\text{ee} |\mathbf{r}_1 - \mathbf{r}_2|}
\end{equation*}
\end{document}