Thank you for reply. My question mostly concern a theoretical aspect rather than practical one. To be not misunderstood, what is the mathematical model that one should apply to be able to deal with twinned intensities with their errors? I mean, I+_what? I ask this In order to state some general consideration on the accuracy about the recovery the true intensities on varying of alpha.
Thanks
Fulvio
Fulvio Saccoccia PhD
Dept. of Biochemical Sciences
Sapienza University of Rome
5, Piazzale A. Moro 00185
phone +39 0649910556
----Messaggio Originale----
Da: [log in to unmask]
Inviato: 06/11/2013, 17:25
A: [log in to unmask]
Oggetto: [ccp4bb] AW: [ccp4bb] uncertainites associated with intensities from twinned crystals
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:[log in to unmask]] Im Auftrag von Fulvio Saccoccia
Gesendet: Mittwoch, 6. November 2013 16:58
An: [log in to unmask]
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)
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Fulvio Saccoccia, PhD
Dept. of Biochemical Sciences "A. Rossi Fanelli"
Sapienza University of Rome
Tel. +39 0649910556
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