Clash between \footnote{} and \widetilde{} in revtex document class

Question

I am writing a research paper using the revtek4-2 (or revtex4-1) document class, and I need to write a footnote containing a mathematical symbol that requires \widetilde (not \tilde). It seems that the \footnote{} environment clashes with \widetilde{}, as can be seen from the following minimal working example. Any idea on how to circumvent this issue?

Thank you!

Note 1: if I replace \widetilde{} with \tilde{} everything works, but \tilde{} is not ideal in my case, I really need to use \widetilde{} instead.

Note 2: this issue does not occur for other document classes such as "article", but for publication reasons I have to stick with the revtex4-2 (or revtex4-1) document class.

Note 3: I am using Overleaf to compile the .tex source.

\documentclass[amsmath,amssymb,reprint]{revtex4-2}

\begin{document}

Here is an example of a footnote with ``widetilde" \footnote{Let us try: $\widetilde{\mathcal{M}_{ij}}$}

\end{document}

Answer 1

Whenever there is a "fragile command in moving argument" error you can apply \protect to prevent premature expansion. Unrelated but '' not ".

You will need to remove the broken .bbl file from earlier attempt then run pdflaex, bibtaex, pdflatex, pdflatex.

\documentclass[amsmath,amssymb,reprint]{revtex4-2}

\begin{document}

Here is an example of a footnote with 
``widetilde''\footnote{Let us try: $\protect\widetilde{\mathcal{M}_{ij}}$}

\end{document}

Answer 2

The commands \widetilde and \widehat are redefined by amsfonts in order to use also wider variants, but the package fails to make them robust like the kernel commands.

If you have a few of such cases, add \protect in front of \widetilde or \widehat when in moving arguments.

For a more practical solution, make the command robust. It requires removing the kernel definitions, though.

\documentclass[amsmath,amssymb,reprint]{revtex4-2}

%% remove the kernel inner definition of \widetilde and \widehat
\ExpandArgs{c}\let{widetilde }\relax
\ExpandArgs{c}\let{widehat }\relax
% make them robust
\MakeRobust\widetilde
\MakeRobust\widehat

\begin{document}

Here is an example of a footnote with
``widetilde''\footnote{Let us try: $\widetilde{\mathcal{M}_{ij}}$}

\end{document}

If you have an older LaTeX version which complains about \ExpandArgs, you can replace the two lines using it with

\expandafter\let\csname widetilde \endcsname\relax
\expandafter\let\csname widehat \endcsname\relax

The spaces are important.