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Well there are no general twinning operators for a monoclinic space group (e.g. like P3, P4) but there are two special cases which come to mind: * beta=90 in which case the twinning operator is something like (h,-k,-l) - pseudo primitive orthorhombic. * where the monoclinic lattice lengths conspire to make your lattice pseudo C-centered orthorhombic, in which case the twinning operator is usually something like (h,k,-h-l) but not necessarily identical to that depending on your cell. I've had both of these happen to me. Best way is to run CNS or mmtbx.xtriage (the latter useful for tests other than twinning), both of which list the twin operators that they try. Phil Jeffrey Princeton, NJ siva charan wrote: > hello > > I have a data set to 2.4 A, which has space group of *P21 *. From the > cumulative intensity distribution graph, indicates that data is > twinned . To run the detwin program in ccp4, one has to know the > twinning operator. Can anybody help me, how can I get the twinning > operator for monoclinic space group. I have checked for twin operators > at http://www.ccp4.ac.uk/dist/html/twinning.html, but there are no > twining operators for monoclinic space group. > regards > siva