\documentclass[preview]{standalone}
\usepackage{varwidth}
\usepackage[utf8x]{inputenc}
\usepackage{amsmath, amssymb, graphics, setspace}

\begin{document}
\begin{varwidth}{50in}
  \begin{equation}
    {J}\left( r_{ij} \right) = 4 a \left( \frac{r_{ij}}{d}  \right)^2 \left( 1 - b \left( \frac{r_{ij}}{d}  \right)^2 \right) e^{-\left( \frac{r_{ij}}{d}  
    \right)^2 }\Theta (R_c - r_{ij}) \nonumber
  \end{equation}
\end{varwidth}
\end{document}
