Dear Fulvio, you cannot detwin perfectly twinned data with this formula. The term (1-2α) becomes zero, so you are dividing by zero. With good refinement programs (ShelX, Refmac), refinement is done against twinned data, which is better than to detwin the data with the formula you mention.
As I understand it, to get map coefficients, the calculated contribution of the twin domain (Fcalc’s) is substracted from Fobs (with the appropriate weighting factors), so what you see in coot is detwinned electron density. In practical terms, the only thing you have to do is to specify the TWIN keyword in Refmac. Best regards, Herman Von: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] Im Auftrag von Fulvio Saccoccia Gesendet: Mittwoch, 6. November 2013 16:58 An: CCP4BB@JISCMAIL.AC.UK Betreff: [ccp4bb] uncertainites associated with intensities from twinned crystals Dear ccp4 users a question about the recovering of true intensities from merohedral twinned crystal. Providing alpha and the twin operator one should be able to recover the intensities from the formulas: I(h1) = (1-α)Iobs(h1)-αIobs(h2)/(1-2α) I(h2) = -αIobs(h1)+(1+α)Iobs(h2)/(1-2α) as stated in many papers and books*. However I was wondering about the uncertainties associated to these measurements, I mean: for all physical observable an uncertainty should be given. Hence, what is the uncertainty associated to a perfect merohedrally twinned crystal (alpha=0.5)? It is clear that in this case we drop in a singular value of the above formulas. Please, let me know your hints or your concerns on the matter. Probably there is something that it is not so clear to me. Thanks in advance Fulvio ref. **(C. Giacovazzo, H. L. Monaco, G. Artioli, D. Viterbo, M. Milaneso, G. Ferraris, G. Gilli, P. Gilli, G. Zanotti and M. Catti. Fundamentals of Crystallography, 3rd edition. IUCr Texts on Crystallography No. 15, IUCr/Oxford University Press, 2011; Chandra, N., Acharya, K. R., Moody, P. C. (1999). Acta Cryst. D55. 1750-1758) -- Fulvio Saccoccia, PhD Dept. of Biochemical Sciences "A. Rossi Fanelli" Sapienza University of Rome Tel. +39 0649910556
