$RDF group (relevant for RUNTYP=MD)
This group defines the pairs of atoms for which the
radial distribution functions are to be computed, at the
end of a molecular dynamics trajectory. The input is
similar in style to $EFRAG, consisting of separate lines,
with the word STOP ending each particular pair.
Line 1. NRDF=
gives the number of RDFs which should be computed.
gives a string for the printout (a good choice involves
both atoms, such as ClCl), the name of the $FRAGNAME
containing the first atom of the pair, the name of the
$FRAGNAME input group with the second atom of the pair, and
how many such pairs exist.