Many monomers have a torsion periodicity given
Here is an extract from GLU.cif
loop_
_chem_comp_tor.comp_id
_
chem_comp_tor.id_chem_comp_tor.atom_id_1
_chem_comp_tor.atom_id_2
_chem_comp_tor.atom_id_3
_chem_comp_tor.atom_id_4
_chem_comp_tor.value_angle
_chem_comp_tor.value_angle_esd
_chem_comp_tor.period
GLU chi1 N CA CB CG 180.000 15.000 3
GLU chi2 CA CB CG CD 180.000 15.000 3
GLU chi3 CB CG CD OE2 0.000 30.000 2
The chi1 and chi2 have targets 180 or 60 or -60 (ie periodicity 3
chi3 targets 0 or 180 - periodcity 2.
Look at you dictionary for you compound and check periodicity..
If it is already set to 2 or 3 and you drag the ligand into place COOT will then refine it to the nearest of the target values.
Is that explanation comprehensible?
Eleanor