\textsuperimposetilde does not superimpose as \sum subscript

Question

Compiling the document, the tilde and P lie seperately under the \sum operator, but as the command superimpose would suggest, I would like them to be superimposed. Works just fine when using \textsuperimposetilde{P} as subscript for alphabetical characters.

\documentclass[11pt]{article}
\usepackage{geometry}                
\geometry{letterpaper}                   

\usepackage{graphicx}
\usepackage{amssymb}
\usepackage{epstopdf}
\usepackage{amsmath}
\usepackage{tikz-cd}
\usepackage{tikz}
\usetikzlibrary{matrix,arrows,decorations.pathmorphing}
\newcounter{qcounter}
\usepackage{fontenc}
\usepackage{marvosym}
\usepackage{wrapfig}
\usepackage{harpoon}
\usepackage{textcomp}
\usepackage{MnSymbol}
\usepackage{mathtools}
\usepackage{xfrac}
\usepackage[safe]{tipa}

\newcommand\sit[1]{%
\textsuperimposetilde{#1}}

\begin{document}

\[
 \not{\text{S}}_{hkl}\, =\,  \sum_{\sit{P}}\, f_{{\sit{P}}_{a}}\, e^{[-i2\pi (hu_{p}+kv_{p}+ lw_{p}]}
 \]

\end{document}

Answer 1

There should be some better method than this. \textsuperimposetilde{P} is used in text mode. Try this

\newcommand\sit[1]{%
\text{\textsuperimposetilde{\ensuremath {#1}}}}

for math mode.

Update: As suggested by @egreg, using

\newcommand\sit[1]{% 
\text{\smash[b]{\textsuperimposetilde{$#1$}}}}

will give better results in the placement of subscripts (Watch out for the placement of a).

Full code:

\documentclass[11pt]{article}
\usepackage{geometry}
\geometry{letterpaper}

\usepackage{graphicx}
\usepackage{amssymb}
\usepackage{epstopdf}
\usepackage{amsmath}
\usepackage{tikz-cd}
\usepackage{tikz}
\usetikzlibrary{matrix,arrows,decorations.pathmorphing}
\newcounter{qcounter}
\usepackage{fontenc}
\usepackage{marvosym}
\usepackage{wrapfig}
\usepackage{harpoon}
\usepackage{textcomp}
\usepackage{MnSymbol}
\usepackage{mathtools}
\usepackage{xfrac}
\usepackage[safe]{tipa}

\newcommand\sit[1]{% 
\text{\smash[b]{\textsuperimposetilde{$#1$}}}}

\begin{document}

\[
 \not{\text{S}}_{hkl}\, =\,  \sum_{\sit{P}}\, f_{{\sit{P}}_{a}}\, e^{[-i2\pi (hu_{p}+kv_{p}+ lw_{p}]}
 \]
\end{document}