"already defined" error within mdframed

Question

Here is a custom theorem environment that utilizes mdframed:

\documentclass{article}

\usepackage[margin=1in]{geometry}
\usepackage[framemethod=tikz]{mdframed}
\usepackage{ifthen}

%theorembox style
\mdfdefinestyle{theorembox}{%
    leftmargin=2em,
    rightmargin=2em,
    innertopmargin=8pt,
    innerbottommargin=8pt,
    roundcorner=8pt,
    backgroundcolor=gray!15
}

%theorem
\newenvironment{theorem}[1][]{%
    \newcounter{thm}
    \setcounter{thm}{1}
    \newsavebox{\thmargbox}
    \savebox{\thmargbox}{#1}
    \begin{list}{}{%
        \setlength{\leftmargin}{0pt}
        \setlength{\rightmargin}{0pt}
    }       
        \item
        \begin{mdframed}[style=theorembox]
            {\large Theorem \arabic{thm}\quad\ifthenelse{\wd\thmargbox=0}{}{#1\par}}%
}{%     
        \end{mdframed}
    \end{list}
    \stepcounter{thm}
}

\begin{document}

\begin{theorem}[First Theorem] 
  If $a\sim b$, then $b\sim a$.
\end{theorem}

%\begin{theorem}[Second Theorem] 
%  If $a\sim b$ and $b\sim c$, then $a\sim c$.
%\end{theorem}

\end{document}

However, if the second theorem is applied, the following error is reported:

! LaTeX Error:  Command \c@thm already defined.

I think this may have more to do with my counter than mdframed, but any assistance would be appreciated.

Answer 1

You have to move \newcounter{thm} and to replace \setcounter{thm}{1} with \refstepcounter{thm}, removing \stepcounter{thm} at the end.

But there are much better ways to do what you want with tcolorbox and the standard theorem environments.

\documentclass{article}
\usepackage{amsthm}
\usepackage[many]{tcolorbox}

\newtheoremstyle{largebreak}
  {}% use the default space above
  {}% use the default space below
  {\normalfont}% body font
  {}% indent (0pt)
  {\large}% header font
  {}% punctuation
  {\newline}% break after header
  {\thmname{#1}\thmnumber{ #2}\thmnote{\quad #3}}% header spec

\theoremstyle{largebreak}
\newtheorem{theorem}{Theorem}
\tcolorboxenvironment{theorem}{
  colback=gray!15,
  boxrule=0.4pt,
}

\begin{document}

\begin{theorem}[First Theorem]
  If $a\sim b$, then $b\sim a$.
\end{theorem}

\begin{theorem}[Second Theorem]
  If $a\sim b$ and $b\sim c$, then $a\sim c$.
\end{theorem}

\begin{theorem}% no optional argument
$1+1=2$, no question.
\end{theorem}

\end{document}

You can get a similar result with mdframed:

\documentclass{article}
\usepackage{amsthm}
\usepackage[framemethod=tikz]{mdframed}

\newtheoremstyle{largebreak}
  {}% use the default space above
  {}% use the default space below
  {\normalfont}% body font
  {}% indent (0pt)
  {\large}% header font
  {}% punctuation
  {\newline}% break after header
  {\thmname{#1}\thmnumber{ #2}\thmnote{\quad #3}}% header spec

\theoremstyle{largebreak}
\newmdtheoremenv[
    leftmargin=2em,
    rightmargin=2em,
    innertopmargin=2pt,
    innerbottommargin=8pt,
    roundcorner=8pt,
    backgroundcolor=gray!15
]{theorem}{Theorem}

\begin{document}

\begin{theorem}[First Theorem] 
  If $a\sim b$, then $b\sim a$.
\end{theorem}

\begin{theorem}[Second Theorem] 
  If $a\sim b$ and $b\sim c$, then $a\sim c$.
\end{theorem}

\begin{theorem}% no optional argument
$1+1=2$, no question.
\end{theorem}

\end{document}