\documentclass[preview]{standalone}
\usepackage{varwidth}
\usepackage[utf8x]{inputenc}
\usepackage{amsmath,amssymb,amsthm,bm}
\begin{document}
\begin{varwidth}{50in}
  \begin{equation}
    \bm{H}_{aniso}  = -\sum_{{ i}=1}^{N} K_{an}(\bm{r}_{i})\, \left( \vec{s}_{i} \cdot \vec{n}_{i} \right)^2, \nonumber
  \end{equation}
\end{varwidth}
\end{document}
