3.5 Additional Input Sections

3.5.17 Isotopic Substitutions ($isotopes)

For vibrational frequency calculations (JOBTYPE = FREQ), nuclear masses are set by default to be those corresponding to the most abundant naturally-occurring isotopes. Alternative masses for one or more nuclei can be requested by setting ISOTOPES = TRUE in the $rem section, in which case the $isotopes section is used to specify the desired masses as described in Section 11.10.2. Isotopic substitutions incur negligible additional cost in a frequency calculation.