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Short helpfile for action KDE
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The following arguments are compulsory: 

               GRID_MIN - ( default=auto ) the lower bounds for the grid 
               GRID_MAX - ( default=auto ) the upper bounds for the grid 
                 METRIC - the inverse covariance to use for the kernels that are 
                          added to the grid 
                 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 
                 KERNEL - ( default=GAUSSIAN ) the kernel function you are using. 
                          More details on the kernels available in plumed plumed can be 
                          found in \ref kernelfunctions. 
          CONCENTRATION - the concentration parameter for Von Mises-Fisher 
                          distributions (only required for SPHERICAL_KDE) 

In addition you may use the following options: 

                 SERIAL - ( default=off ) do the calculation in serial. Do not 
                          parallelize 
 IGNORE_IF_OUT_OF_RANGE - ( default=off ) if a kernel is outside of the range of the 
                          grid it is safe to ignore 
                    ARG - the input for this action is the scalar output from one or 
                          more other actions. The particular scalars that you will use are 
                          referenced using the label of the action. If the label appears on its 
                          own then it is assumed that the Action calculates a single 
                          scalar value. The value of this scalar is thus used as the input 
                          to this new action. If * or *.* appears the scalars calculated 
                          by all the proceeding actions in the input file are taken. 
                          Some actions have multi-component outputs and each component of 
                          the output has a specific label. For example a \ref DISTANCE 
                          action labelled dist may have three components x, y and z. To take 
                          just the x component you should use dist.x, if you wish to take 
                          all three components then use dist.*.More information on the 
                          referencing of Actions can be found in the section of the manual on the 
                          PLUMED \ref Syntax. Scalar values can also be referenced using 
                          POSIX regular expressions as detailed in the section on \ref 
                          Regex. To use this feature you you must compile PLUMED with the 
                          appropriate flag.. You can use multiple instances of this keyword i.e. 
                          ARG1, ARG2, ARG3... 
                HEIGHTS - this keyword takes the label of an action that calculates a 
                          vector of values. The elements of this vector are used as weights 
                          for the Gaussians. 
                VOLUMES - this keyword take the label of an action that calculates a 
                          vector of values. The elements of this vector divided by the 
                          volume of the Gaussian are used as weights for the Gaussians 
              BANDWIDTH - the bandwidths for kernel density esimtation 
               GRID_BIN - the number of bins for the grid 
           GRID_SPACING - the approximate grid spacing (to be used as an alternative 
                          or together with GRID_BIN) 


