why when i comment \usepackage{geometry} i get error?

\documentclass{book}

\usepackage{geometry}

\makeatletter
\@ifpackageloaded{geometry}
 {                               % i the package was loaded 
    \newcommand*{\qw}{\itshape}  % this is enabled, else is not enabled.                                                                                               
 }{}
\makeatother

\begin{document}
\qw hallo
\end{document}



Question: Quantum Mechanical Meaning of Atomic Orbitals
Answer: While it's possible for the atom to be in a superposition on energy states, the superposition collapses to an eigenstate as soon as we observe it. Therefore every atom we observe will have a distinct electronic configuration.

Incidentally, note that for a multi electron atom the assignment of electrons to the atomic orbitals 1s, 2s, etc, is an approximation. The electron correlation mixes up the atomic orbitals so strictly speaking we don't have separate atomic orbitals, just a single wavefunction for the atom. However, in most cases using atomic orbitals is an excellent approximation.