当我尝试\ce{CH4}在标题中使用 documentclass beamer 时，出现Undefined control sequence错误。
以下是（几乎）最小（不）工作示例：
\documentclass{beamer}
\listfiles
\usepackage{mhchem}
\title{\ce{CH4}}
\begin{document}
\begin{frame}
\ce{CH4}
\end{frame}
\end{document}
运行后pdflatex mini，这是写入 stdout+stderr 的输出（mini.log对于 Stack Exchange 帖子来说太长了）。有什么解决方法或其他方法可以用来在标题中排版化学公式吗？
（我的实际的用例在海报的标题中）
This is pdfTeX, Version 3.1415926-2.5-1.40.14 (TeX Live 2013)
restricted \write18 enabled.
entering extended mode
(./mini.tex
LaTeX2e <2011/06/27>
Babel <3.9h> and hyphenation patterns for 78 languages loaded.
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Document Class: beamer 2013/12/02 3.33 A class for typesetting presentations (r
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Package mhchem Warning: You did not specify a 'version' option for the mhchem
(mhchem) package. If you are about to create a new document,
(mhchem) please write \usepackage[version=3]{mhchem} in your
(mhchem) preamble in order to use the most recent version of
(mhchem) mhchem.
)
! Undefined control sequence.
\mhchem@ce ...ate {s}\chardef \mhchem@ce@substate
=0\relax \mhchem@ce@result...
l.4 \begin{document}
? No pages of output.
Transcript written on mini.log.
答案1
\ce如果您在之后才定义标题，则标题中有效\begin{document}。此外，软件包作者强烈建议您[version=3]在加载软件包时使用该选项。
\documentclass{beamer}
\usepackage[version=3]{mhchem}
\begin{document}
\title{\ce{CH4}}
\titlepage
\begin{frame}
\ce{CH4}
\end{frame}
\end{document}