igroup {<igroup-statement>} end

<igroup-statement> :==

  interaction <atom-selection> <atom-selection> 
              [weight { <energy-term> <real> } end]
  --{Compute interaction terms between the specified sets of atoms
     during all energy calculations:
     - for pairwise interactions (i-j) i in a and j in b
     - for angles (i-j-k), i,j,k have to be in a or b with at least
       one component in a and at least one in b.
     - for dihedrals & impropers (i-j-k-l), i,j,k,l have to be in a or b 
       with at least one component in a and at least one in b.
     Each energy term can be weighted by a weighting factor.}