\documentclass[preview]{standalone}
\usepackage{varwidth}
\usepackage[utf8x]{inputenc}
\usepackage{amsmath,amssymb,amsthm,bm}
\begin{document}
\begin{varwidth}{50in}
  \begin{equation}
    \bm{H}_{zeeman} = -\mu_{B}\mu_0\sum_{i=0}^{N}g_{i} \vec{s}_{i} \cdot \vec{H}_{ext} \nonumber 
  \end{equation}
\end{varwidth}
\end{document}
