The real-space residual RSR, as described by Branden & Jones
(1990), evaluated over all atoms in the base moiety of the
nucleic acid monomer.
sum|pobs - pcalc|
RSR = ---------------------
sum|pobs + pcalc|
pobs = the density in an 'experimental' map
pcalc = the density in a 'calculated' map
sum is taken over the specified grid points
Details of how these maps were calculated should be given
in _struct_mon_details.RSR. The sums are taken over all map grid
points near the relevant atoms. The radius for including grid
points in the calculation should also be given in
_struct_mon_details.RSR.
Ref: Branden, C.-I. & Jones, T. A. (1990). Nature (London), 343,
687-689.