Using \hbar from MLModern after loading AMSfonts

Question

As the title suggests, I am trying to use \hbar in conjunction with the mlmodern and amsfonts packages. I need to use the \hbar provided by mlmodern, but when amsfonts is loaded, I get the original CM/LM.

I have tried the following, which results in an undefined control sequence error for $\hbar$:

\documentclass{article}
\usepackage{mlmodern}
\let\mlmhbar\hbar
\usepackage{amsfonts}
\let\amshbar\hbar
\begin{document}
\def\hbar{\mlmhbar}
$\hbar$
\end{document}

Any advice is appreciated.

Answer 1

You need to use \NewCommandCopy, because \hbar is defined as a robust command.

\documentclass{article}
\usepackage{mlmodern}
\NewCommandCopy{\mlmhbar}{\hbar}
\usepackage{amsfonts}
\NewCommandCopy{\amshbar}{\hbar}% just for the comparison
\RenewCommandCopy{\hbar}{\mlmhbar}
\begin{document}
$h\hbar$ $h\amshbar$
\end{document}

However, you can see that the bar with MLModern hits the serif of “h”.

\documentclass{article}
\usepackage{mlmodern}
\usepackage{amsfonts}
\NewCommandCopy{\amshbar}{\hbar}% just for the comparison
\makeatletter
\RenewDocumentCommand{\hbar}{}{%
  {\mathpalette\hbar@bar\relax\mkern -9muh}%
}
\newcommand{\hbar@bar}[2]{\raisebox{-0.075\height}{$\m@th#1\mathchar'26$}}
\makeatother
\begin{document}
$h\hbar$ $h\amshbar$
$\scriptstyle h\hbar$
\end{document}