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Short helpfile for action COM
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The input trajectory can be in any of the following formats: 

                  ATOMS - the list of atoms which are involved the virtual atom's 
                          definition. For more information on how to specify lists of atoms see 
                          \ref Group 

The following options are available

                  NOPBC - ( default=off ) ignore the periodic boundary conditions 
                          when calculating distances 
                   MASS - ( default=off ) If set center is mass weighted 
                 PHASES - ( default=off ) Compute center using trigonometric phases 
                WEIGHTS - Center is computed as a weighted average. 
             SET_CHARGE - Set the charge of the virtual atom to a given value. 
               SET_MASS - Set the mass of the virtual atom to a given value. 


