16.7 Redefining and printing atomic masses

The current masses of all atoms can be printed using

MASS,PRINT

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. If INIT is given, all previous mass definitions are deleted and the defaults are reset.

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 follow each other. The last given mass is used.

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; ...}