Redefined `epsilon` to `varepsilon`

Question

MWE:

% !TeX program = xelatex
\documentclass{article}
    \usepackage{libertinus}
    \renewcommand{\epsilon}{\varepsilon}

\begin{document}
    This is \texttt{varepsilon}: $\varepsilon$.

    This is \texttt{epsilon}, which should be looked the same as \texttt{varepsilon} $\epsilon$.
\end{document}

I'm using XeLaTeX. I can't redefine epsilon to varepsilon, as in the picture above. However, if I switch to pdfLaTeX, this does not happen.

For some reason, I have to use XeLaTeX. How can I modify my code to get the variant ε when I enter epsilon? Thanks for your help.

Answer 1

You need to move the instruction \renewcommand{\epsilon}{\varepsilon} to after the \begin{document} directive. This is necessary because the libertinus package, if used with XeLaTeX or LuaLaTeX, defers some of its setup work until the end of the preamble is reached.

% !TeX program = xelatex
\documentclass{article}
\usepackage{libertinus}  

\begin{document}
\renewcommand{\epsilon}{\varepsilon}

varepsilon: $\varepsilon$ \quad epsilon: $\epsilon$
\end{document}

Answer 2

Since libertinus-otf uses unicode-math, you need to delay the setting.

\documentclass{article}
\usepackage{libertinus}

\AtBeginDocument{\renewcommand{\epsilon}{\varepsilon}}

\begin{document}

This is \texttt{varepsilon}: $\varepsilon$.

This is \texttt{epsilon}, which should be looked the same as 
\texttt{varepsilon} $\epsilon$.

\end{document}