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Short helpfile for action PIV
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The following arguments are compulsory: 

                 SWITCH - The switching functions parameter.You should specify a 
                          Switching function for all PIV blocks.Details of the various 
                          switching functions you can use are provided on \ref 
                          switchingfunction. You can use multiple instances of this keyword i.e. 
                          SWITCH1, SWITCH2, SWITCH3... 
              PRECISION - the precision for approximating reals with integers in 
                          sorting. 
               REF_FILE - PDB file name that contains the \f$i\f$th reference 
                          structure. 
               PIVATOMS - Number of atoms to use for PIV. 
                   SORT - Whether to sort or not the PIV block. 
              ATOMTYPES - The atom types to use for PIV. 

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 
                   TEST - ( default=off ) Print the actual and reference PIV and exit 
                    COM - ( default=off ) Use centers of mass of groups of atoms 
                          instead of atoms as specified in the Pdb file 
              ONLYCROSS - ( default=off ) Use only cross-terms (A-B, A-C, B-C, ...) 
                          in PIV 
             ONLYDIRECT - ( default=off ) Use only direct-terms (A-A, B-B, C-C, ...) 
                          in PIV 
            DERIVATIVES - ( default=off ) Activate the calculation of the PIV for 
                          every class (needed for numerical derivatives). 
                  NLIST - ( default=off ) Use a neighbor list for distance 
                          calculations. 
                 SERIAL - ( default=off ) Perform the calculation in serial - for 
                          debug purpose 
                  TIMER - ( default=off ) Perform timing analysis on heavy loops. 
                SFACTOR - Scale the PIV-distance by such block-specific factor 
                 VOLUME - Scale atom-atom distances by the cubic root of the cell 
                          volume. The input volume is used to scale the R_0 value of the 
                          switching function. 
              UPDATEPIV - Frequency (in steps) at which the PIV is updated. 
              NL_CUTOFF - Neighbor lists cutoff. 
              NL_STRIDE - Update neighbor lists every NL_STRIDE steps. 
                NL_SKIN - The maximum atom displacement tolerated for the neighbor 
                          lists update. 


