Dear George, Yes, we’ve developed new likelihood functions that work with intensity data. They’re already available for the molecular replacement calculations in recent nightly-build versions of Phaser (though it’s been a while since a new nightly was released, and we’ve fixed a few problems with outliers that were more extreme than we had anticipated encountering). I’ll be presenting our work on the intensity-based SAD likelihood target at the upcoming CCP4 Study Weekend.
It’s possible to define exact intensity-based likelihood functions (at least “exact” when the measurement errors are Gaussian in the observed intensity), but we haven’t found a way of evaluating them without either numerical integration (expensive) or approximation. However, we’ve got a new approximation that turns out to be excellent over the whole range from small to extremely large intensity errors, and which is very efficient to work with. Best wishes, Randy On 3 Dec 2014, at 15:07, George M. Sheldrick <gshe...@shelx.uni-ac.gwdg.de> wrote: > Dear Randy, > > I could not agree more. Statistical methods for phasing and refinement > must be better using the observed intensities and their esds than with > (c)truncated F-values. In particular one should merge intensities, not > truncated Fs! > > To elaborate on Harry's comment, when SHELXL started refining only > against intensities 22 years ago, I received many complaints from irate > small molecule crystallographers whose papers had been rejected because > the unweighted R-factors R2 (based on intensities) were too high. I even > sent a letter to editors of the journals involved to explain why > R-factors based on intensities are at least twice as high as those based > on F, but to no avail. So I had to output R1 (the unweighted R-value > based on F) even though the structure had been refined against > intensitites, then everyone was happy. > > Do I correctly understand that you have developed new (better) maximum > likelihood criteria for use with I rather than F? > > Best wishes, George > > > > On 12/02/2014 10:26 PM, Randy Read wrote: >> Dear Mohamed, >> >> At the moment, a lot of programs require amplitudes, but I believe that they >> should all eventually be updated to use intensities. In fact, we’re in the >> end stages of a large project to switch Phaser from using amplitudes to >> using intensities. There are a number of reasons why, in principle, it’s >> better to work in terms of intensities. One is that it’s perfectly >> reasonable to have a negative observed intensity, which can come from >> subtracting a background estimate with measurement errors from a very weak >> peak with its own measurement errors. That, of course, is where the French >> and Wilson algorithm comes in, allowing an amplitude to be estimated without >> simply taking a square root. However, the problem with the French and >> Wilson algorithm is that it loses information, i.e. you can’t reconstruct >> the intensity and its standard deviation. What you get out of French & >> Wilson depends on the estimate of the expected intensity for a reflection, >> which is typically taken from t > he mean in the resolution shell but should vary with direction for crystals > suffering from anisotropic diffraction and should be modulated for crystals > with translational non-crystallographic symmetry. >> >> Another reason it’s better to work in terms of intensities is that it’s >> reasonable to assume that the measurement errors for intensities are >> Gaussian, but then less reasonable to assume that for amplitudes >> (particularly with the problem that amplitudes can’t be negative). >> >> For now, you need amplitudes for a lot of purposes and then the French & >> Wilson algorithm is useful. But what I would strongly recommend is that you >> hang on to the intensities and you make sure that the intensities are >> deposited at the PDB. It’s a pity that many PDB depositions only have >> amplitudes that have been through French & Wilson, so that new procedures >> based on intensities won’t be able to be applied with their full power. >> >> Best wishes, >> >> Randy Read >> >> ----- >> Randy J. Read >> Department of Haematology, University of Cambridge >> Cambridge Institute for Medical Research Tel: +44 1223 336500 >> Wellcome Trust/MRC Building Fax: +44 1223 336827 >> Hills Road >> E-mail: rj...@cam.ac.uk >> Cambridge CB2 0XY, U.K. >> www-structmed.cimr.cam.ac.uk >> >> On 1 Dec 2014, at 20:49, Mohamed Noor <mohamed.n...@staffmail.ul.ie> wrote: >> >>> Dear crystallographers >>> >>> Is there any reason for using one data type over the other? Are there any >>> errors associated with the French and Wilson I-to-F conversion step? >>> >>> Thanks. >>> Mohamed >> > > -- > Prof. George M. Sheldrick FRS > Dept. Structural Chemistry, > University of Goettingen, > Tammannstr. 4, > D37077 Goettingen, Germany > Tel. +49-551-39-33021 or -33068 > Fax. +49-551-39-22582 ------ Randy J. Read Department of Haematology, University of Cambridge Cambridge Institute for Medical Research Tel: + 44 1223 336500 Wellcome Trust/MRC Building Fax: + 44 1223 336827 Hills Road E-mail: rj...@cam.ac.uk Cambridge CB2 0XY, U.K. www-structmed.cimr.cam.ac.uk
