-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Dear Fulvio,
with simple error propagation, the error would be sigma(I(h1)) = (1-α)sigma(Iobs(h1))-α*sigma(Iobs(h2))/(1-2α) would it not? Although especially for theoretical aspects you should be concerned about division by zero. Best, Tim On 11/06/2013 05:54 PM, Fulvio Saccoccia wrote: > 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: herman.schreu...@sanofi.com > Inviato: 06/11/2013, 17:25 A: CCP4BB@JISCMAIL.AC.UK 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: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 > - -- Dr Tim Gruene Institut fuer anorganische Chemie Tammannstr. 4 D-37077 Goettingen GPG Key ID = A46BEE1A -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.15 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iD8DBQFSewyuUxlJ7aRr7hoRAnP/AJ9biIGWJsHnbcQ4IaG5fusZTN0iOgCfR6bu 3po97EKDH98F881dH3CgD2M= =2QkU -----END PGP SIGNATURE-----
