[Tex/LaTex] Undefined Control Sequence occurs at: \begin{document}

document-classeserrors

I wasn't having any problem with my .tex file, but after compiling a separate document – which used a different document class – my current file no longer loads.

Specifically, I was using the imsart document class, compiled a separate file in the biom class, and now my imsart file won't load. The error I'm getting is: 

(c:/Program Files/PCTeX/PCTeXv6/texmf-dist/tex/latex/lm/ot1lmss.fd)
! Undefined control sequence.
<argument> \bibsep 

 l.66 \begin{document}

I've tried moving the \begin{document} around but that doesn't do anything. I'm almost 100% sure it has something to do with the natbib package, but I can't figure out what. What's bothersome is that no edits were made to the file, and it was just after compiling the other class file that my file won't work.

I've checked to make sure I'm compiling a LaTeX pdf file, as I was before.

The basic code is here, which gives the same error, is virtually copied from the imsart class website: 

\documentclass[aoas,preprint]{imsart}
\RequirePackage[OT1]{fontenc}
\RequirePackage{amsthm,amsmath}
\RequirePackage[numbers]{natbib}
\RequirePackage[colorlinks,citecolor=blue,urlcolor=blue]{hyperref}
\startlocaldefs
\numberwithin{equation}{section}
\theoremstyle{plain}
\newtheorem{thm}{Theorem}[section]
\endlocaldefs
\begin{document}
\section{Ordinary text}
  It is interesting that simulations with the \cite{r4} model worked well
 \begin{thebibliography}{9}
 \bibitem{r4}
\textsc{Prokhorov, Yu.} (1956).
Convergence of random processes and limit theorems in probability
theory. \textit{Theory  Probab.  Appl.}
\textbf{1} 157--214.
\MR{84896}
\end{thebibliography}
\end{document}

Best Answer

I just had this same problem, and found the solution.

For me, it turned out to be a clash between two packages: Apparently I was loading both the subfigure & subfig packages, which are not compatible (according to this hint).

If I comment out either

\usepackage{subfigure}

OR

\usepackage{subfig}

The code executes properly. (subfigure is apparently obsolete and should be replaced by subfig)

Apparently You should only use one or the other, as they theoretically provide similar functionality, while I was previously loading both one after the other.

Perhaps your class imsart.cls is importing both packages?

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