# Pgfplots, declare function and evaluation of parameters

I just want to plot a function with some parameters which I want to be arguments of the function Referring to this answer I made my own code with my own function, but when I try to compile it says Undefined control sequence. What am I doing wrong?

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\begin{document}
\begin{tikzpicture}[
declare function={VMorse(\x, \De, \aval, \xre) = \De*(exp(-2*\aval*(\x-\xre))- 2*exp(-\aval*(\x-\xre)));},%
]
\begin{axis}
\end{axis}
\end{tikzpicture}
\end{document}


White spaces! Do not use them in the arguments of the function

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\begin{document}
\begin{tikzpicture}[
declare function={VMorse(\x,\De,\aval,\xre)=\De*(exp(-2*\aval*(\x-\xre))- 2*exp(-\aval*(\x-\xre)));},%
]
\begin{axis}