13.7 Redefining atomic masses

The atomic masses can be redefined using

MASS,[type,][symbol=mass, ...]

The optional keyword type can take either the value AVER[AGE] for using average isotope masses, or ISO[TOPE] for using the masses of the most abundant isotopes. This affects only the rotational constants and vibrational frequencies. As in most quantum chemistry packages, the default for type is AVERAGE.

Individual masses can be changed by the following entries, where symbol is the chemical symbol of the atom and mass is the associated mass. Several entries can be given on one MASS card, and/or several MASS cards can immediately follow each other. If there is any other input between two sets of mass cards, all mass definitions but not type) from the first one are overwritten by the last one.

Note that specifying different isotope masses for symmetry related atoms lowers the symmetry of the system if the molecular centre of mass is taken as the origin. This effect can be avoided by using the charge centre as origin, i.e., specifying CHARGE as first entry in the GEOMETRY input:

GEOMETRY={CHARGE; ...}