I would like to typeset a fraction without the fraction bar. In the following MWE, I try to do this using substack command from the mathtools package. But, as you can see in the output, the spacing is not the same as in the original fraction. How can I typeset a fraction without the fraction bar (not necessarily using mathtools?
\documentclass[oneside,11pt]{book}
\usepackage[semibold,tt=false]{libertine}
\usepackage{libertinust1math}
\usepackage[T1]{fontenc}
\usepackage[
expansion = false ,
tracking = smallcaps ,
letterspace = 40 ,
final
]{microtype}
\usepackage[font={sf,small},labelsep=quad,labelfont=sc]{caption}
\usepackage[subrefformat=parens]{subcaption}
\usepackage{mathtools}
\usepackage{etoolbox}
\begin{document}
$\frac{1}{2}$
$\substack{1\2}$
\end{document}
