$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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generated on 7/7/2017