Q-Chem 4.3 User’s Manual

3.25 Isotopic Substitutions ($isotopes)

By default Q-Chem uses atomic masses that correspond to the most abundant naturally occurring isotopes. Alternative masses for any or all of the atoms in a molecule can be specified using the $isotopes keyword. The ISOTOPES $rem variable must be set to TRUE for this section to take effect. See Section 10.13.6 for details.