How can there be nothing "wrong" with something that is unphysical? Intensities cannot be negative. How could you measure a negative number of photons? You can only have a Gaussian distribution around I=0 if you are using an incorrect, unphysical statistical model. As I understand it, the physics predicts that intensities from diffraction should be gamma distributed (i.e., the square of a Gaussian variate), which makes sense as the gamma distribution assigns probability 0 to negative values.
On Jun 20, 2013, at 1:00 PM, Bernard D Santarsiero <b...@uic.edu> wrote: > There's absolutely nothing "wrong" with negative intensities. They are > measurements of intensities that are near zero, and some will be negative, > and others positive. The distribution around I=0 can still be Gaussian, and > you have true esd's. With F's you used a derived esd since they can't be > formally generated from the sigma's on I, and are very much undetermined for > small intensities and small F's. > > Small molecule crystallographers routinely refine on F^2 and use all of the > data, even if the F^2's are negative. > > Bernie > > On Jun 20, 2013, at 11:49 AM, Douglas Theobald wrote: > >> Seems to me that the negative Is should be dealt with early on, in the >> integration step. Why exactly do integration programs report negative Is to >> begin with? >> >> >> On Jun 20, 2013, at 12:45 PM, Dom Bellini <dom.bell...@diamond.ac.uk> wrote: >> >>> Wouldnt be possible to take advantage of negative Is to >>> extrapolate/estimate the decay of scattering background (kind of Wilson >>> plot of background scattering) to flat out the background and push all the >>> Is to positive values? >>> >>> More of a question rather than a suggestion ... >>> >>> D >>> >>> >>> >>> From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Ian >>> Tickle >>> Sent: 20 June 2013 17:34 >>> To: ccp4bb >>> Subject: Re: [ccp4bb] ctruncate bug? >>> >>> Yes higher R factors is the usual reason people don't like I-based >>> refinement! >>> >>> Anyway, refining against Is doesn't solve the problem, it only postpones >>> it: you still need the Fs for maps! (though errors in Fs may be less >>> critical then). >>> -- Ian >>> >>> On 20 June 2013 17:20, Dale Tronrud >>> <det...@uoxray.uoregon.edu<mailto:det...@uoxray.uoregon.edu>> wrote: >>> If you are refining against F's you have to find some way to avoid >>> calculating the square root of a negative number. That is why people >>> have historically rejected negative I's and why Truncate and cTruncate >>> were invented. >>> >>> When refining against I, the calculation of (Iobs - Icalc)^2 couldn't >>> care less if Iobs happens to be negative. >>> >>> As for why people still refine against F... When I was distributing >>> a refinement package it could refine against I but no one wanted to do >>> that. The "R values" ended up higher, but they were looking at R >>> values calculated from F's. Of course the F based R values are lower >>> when you refine against F's, that means nothing. >>> >>> If we could get the PDB to report both the F and I based R values >>> for all models maybe we could get a start toward moving to intensity >>> refinement. >>> >>> Dale Tronrud >>> >>> >>> On 06/20/2013 09:06 AM, Douglas Theobald wrote: >>> Just trying to understand the basic issues here. How could refining >>> directly against intensities solve the fundamental problem of negative >>> intensity values? >>> >>> >>> On Jun 20, 2013, at 11:34 AM, Bernhard Rupp >>> <hofkristall...@gmail.com<mailto:hofkristall...@gmail.com>> wrote: >>> As a maybe better alternative, we should (once again) consider to refine >>> against intensities (and I guess George Sheldrick would agree here). >>> >>> I have a simple question - what exactly, short of some sort of historic >>> inertia (or memory lapse), is the reason NOT to refine against intensities? >>> >>> Best, BR >>> >>> >>> >>> >>> -- >>> >>> This e-mail and any attachments may contain confidential, copyright and or >>> privileged material, and are for the use of the intended addressee only. If >>> you are not the intended addressee or an authorised recipient of the >>> addressee please notify us of receipt by returning the e-mail and do not >>> use, copy, retain, distribute or disclose the information in or attached to >>> the e-mail. >>> >>> Any opinions expressed within this e-mail are those of the individual and >>> not necessarily of Diamond Light Source Ltd. >>> >>> Diamond Light Source Ltd. cannot guarantee that this e-mail or any >>> attachments are free from viruses and we cannot accept liability for any >>> damage which you may sustain as a result of software viruses which may be >>> transmitted in or with the message. >>> >>> Diamond Light Source Limited (company no. 4375679). Registered in England >>> and Wales with its registered office at Diamond House, Harwell Science and >>> Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom >>> >>> >>> >>> >>> >>> >>> >>> >>> >> >
