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

                  ATOMS - the atoms involved in each of the vectors you wish to 
                          calculate. Keywords like ATOMS1, ATOMS2, ATOMS3,... should be listed 
                          and one vector will be calculated for each ATOM keyword you 
                          specify (all ATOM keywords should specify the indices of two 
                          atoms). The eventual number of quantities calculated by this action 
                          will depend on what functions of the distribution you choose to 
                          calculate. You can use multiple instances of this keyword i.e. ATOMS1, 
                          ATOMS2, ATOMS3... 
                  GROUP - Calculate the distance between each distinct pair of atoms 
                          in the group 
                 GROUPA - Calculate the distances between all the atoms in GROUPA and 
                          all the atoms in GROUPB. This must be used in conjunction with 
                          GROUPB. 
                 GROUPB - Calculate the distances between all the atoms in GROUPA and 
                          all the atoms in GROUPB. This must be used in conjunction with 
                          GROUPA. 

The following arguments are compulsory: 

                     NN - ( default=12 ) The n parameter of the switching function 
                     MM - ( default=0 ) The m parameter of the switching function; 0 
                          implies 2*NN 
                    D_0 - ( default=0.0 ) The d_0 parameter of the switching function 
                    R_0 - The r_0 parameter of the switching function 

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 
                 SWITCH - This keyword is used if you want to employ an alternative 
                          to the continuous switching function defined above. The 
                          following provides information on the \ref switchingfunction that are 
                          available. When this keyword is present you no longer need the NN, MM, 
                          D_0 and R_0 keywords. 
                  VMEAN - calculate the norm of the mean vector. The final value can 
                          be referenced using <em>label</em>.vmean. You can use multiple 
                          instances of this keyword i.e. VMEAN1, VMEAN2, VMEAN3... The 
                          corresponding values are then referenced using <em>label</em>.vmean-1, 
                          <em>label</em>.vmean-2, <em>label</em>.vmean-3... 
                   VSUM - calculate the norm of the sum of vectors. The final value 
                          can be referenced using <em>label</em>.vsum. You can use 
                          multiple instances of this keyword i.e. VSUM1, VSUM2, VSUM3... The 
                          corresponding values are then referenced using <em>label</em>.vsum-1, 
                          <em>label</em>.vsum-2, <em>label</em>.vsum-3... 


