$MASS group (relevant for RUNTYP=HESSIAN, IRC, or DRC)
 
    This group permits isotopic substitution during the
computation of mass weighted Cartesian coordinates.  Of
course, the masses affect the frequencies and normal modes
of vibration.
 
AMASS = An array giving the atomic masses, in amu. The
        default is to use the mass of the most abundant
        isotope.  Masses through element 104 are stored.
 
example - $MASS AMASS(3)=2.0140 $END
will make the third atom in the molecule a deuterium.
 
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