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

                 ORIGIN - we will use the position of this atom as the origin in our 
                          calculation. For more information on how to specify lists of atoms see 
                          \ref Group 

The following arguments are compulsory: 

                   DATA - The multicolvar that calculates the set of base quantities 
                          that we are interested in 
                    DIR - ( default=xyz ) the directions in which we are calculating 
                          the gradient. Should be x, y, z, xy, xz, yz or xyz 
                  NBINS - number of bins to use in each direction for the calculation 
                          of the gradient 
                  SIGMA - ( default=1.0 ) the width of the function to be used for 
                          kernel density estimation 
                 KERNEL - ( default=gaussian ) the type of kernel function to be used 

In addition you may use the following options: 

  NUMERICAL_DERIVATIVES - ( default=off ) calculate the derivatives for these 
                          quantities numerically 
                  NOPBC - ( default=off ) ignore the periodic boundary conditions 
                          when calculating distances 
                 SERIAL - ( default=off ) do the calculation in serial. Do not use 
                          MPI 
                 LOWMEM - ( default=off ) lower the memory requirements 
                TIMINGS - ( default=off ) output information on the timings of the 
                          various parts of the calculation 

