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Hi Jacob, The current version of freerflag in CCP4 already chooses twin proof test sets , do as long as you chose your testset with that algorithm (most CCP4 procedures do) you are fine. Cheers, Robbie Sent with my Windows Phone ________________________________ Van: Keller, Jacob<mailto:[log in to unmask]> Verzonden: 23-7-2015 03:06 Aan: [log in to unmask]<mailto:[log in to unmask]> Onderwerp: Re: [ccp4bb] Rfree handling in Refmac Twin Refinement >If you have twinned data then your Rfree set is already compromised, because pairs of reflections related by the twin law contribute to each other's measured intensity. The operation described is an obvious way to recover from this state. Wouldn't the more rigorous way be to re-solve the structure with an appropriately-selected free set? Or even better, to assign by default twin-proofed free sets from the outset in scaling, as I have heard is done in Phenix? Or maybe late changes in free sets are admittedly not ideal, but don't end up mattering that much in practice? Has someone looked at this? Maybe the structure "forgets" what it had seen in the re-assigned free set after a while? JPK