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

\begin{document}
\begin{varwidth}{50in}
  \begin{equation}
    \mathcal{H}_{\rm long}= 
    -\frac{\mu_{0} \left( \mu_B\right)^2}{4\pi} 
    \sum_{i,j,i\neq j}^{N}
    \frac{g_i g_j}{r_{ij}^3}
    \Big(3 
    \left(\bm{e}_{ij}\cdot \bm{s}_{i}\right) 
    \left(\bm{e}_{ij}\cdot \bm{s}_{j}\right) 
    -\bm{s}_i\cdot\bm{s}_j \Big)
    \nonumber
  \end{equation}
\end{varwidth}
\end{document}
