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

                  GROUP - . For more information on how to specify lists of atoms see 
                          \ref Group 
                 GROUPA - 
                 GROUPB - 

The following arguments are compulsory: 

               GRID_BIN - the number of bins to use when computing the RDF 
                 KERNEL - ( default=GAUSSIAN ) the type of kernel to use for 
                          computing the histograms for the RDF 
                 CUTOFF - ( default=6.25 ) the cutoff at which to stop evaluating the 
                          kernel functions is set equal to sqrt(2*x)*bandwidth in each 
                          direction where x is this number 
                   MAXR - the maximum distance to use for the rdf 
              BANDWIDTH - the bandwidths for kernel density esimtation 
                  CLEAR - ( default=1 ) the frequency with which to clear the 
                          estimate of the rdf. Set equal to 0 if you want to compute an rdf 
                          over the whole trajectory 
                 STRIDE - ( default=1 ) the frequency with which to compute the rdf 
                          and accumulate averages 
                DENSITY - the reference density to use when normalizing the RDF 


