The $QCAUX environment variable determines the directory where Q-Chem searches for auxiliary files and the machine license. If not set explicitly, it defaults to $QC/qcaux.
The $QCAUX directory contains files required to run Q-Chem calculations, including basis set and ECP specifications, SAD guesses (see Chapter 4), library of standard effective fragments (see Section 11.5), and instructions for the AOInts package for generating two-electron integrals efficiently.