\documentclass{article}
\usepackage{a4}
\usepackage{epsf}
\usepackage{graphicx}
\usepackage[round]{natbib}

%\usepackage{multicol}


\def\der#1#2{\frac{\partial #1}{\partial #2}}
\def\dep#1{\left(#1\right)}
\def\depb#1{\left[#1\right]}
\def\dem{1/2}
\def\eq#1{Eq.~\ref{eq:#1}}
\def\fg#1{Fig.~\ref{fg:#1}}
\def\dq{{\dep{\delta q}}}
\def\dt{\delta t}
\def\eg{e.~g.}

%\twocolumn

\begin{document}

\bibliographystyle{plainnat}

\input{tests}

\end{document}