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

\begin{document}
\begin{varwidth}{50in}
  \begin{equation}
    \bm{\omega}_i = 
    \frac{\mu_0 (\mu_B)^2}{4\pi\hbar}\sum_{j}
    \frac{g_i g_j}{r_{ij}^3}
    \, \Big(
    3\,(\bm{e}_{ij}\cdot\bm{s}_{j})\bm{e}_{ij}
    -\bm{s}_{j} \Big) \nonumber
  \end{equation}
\end{varwidth}
\end{document}
