Re: [phenixbb] R value difference in model vs data vs refinement

The R-factor difference of 0.0004 is what you have to expect as the result of writing the coordinates, B-factors, and occupancies to the PDB file. In memory floating-point numbers have >= 12 digits precision (we use double precision for almost everything); in the PDB file you have only 7 digits for the coordinates and just 5 digits for the B-factors and occupancies. Ralf On Fri, Mar 23, 2012 at 9:49 AM, Christian Roth < [email protected]> wrote:

Hi Nat,

I agree with you that the difference is very small and likely negligible. I just asked for curiosity if there might be any reason for this behaviour.

Christian

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On Thu, Mar 22, 2012 at 2:54 PM, Christian Roth

wrote:

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I am not sure I looked into the polygon. In the log it is stated that

Am Donnerstag 22 März 2012 23:07:33 schrieb Nathaniel Echols: the

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R values are calculated after a resolution and sigma cutoff applied. If I understood the log correctly the values taken from the pdb header are without sigma cutoff. Maybe thats the reason for the difference. Does modelvsdata somewhere print the values without cutoff in the log file? I did not find it. However does this mean till firsst OHS in phenix refine a default cutoff is used and in than throughout the refinement no coutoff is used anymore?

After spending some time looking at similar cases today I am not sure myself what is going on. I do not think a sigma cutoff is applied, unless perhaps the PDB header indicates that one was used previously (this is a thoroughly antiquated practice). However, outlier filtering appears to be used throughout. I found one example where nearly 4000 reflections have amplitudes of zero (which is surely not correct), and are discarded as outliers in phenix.refine. This reduces R-free by 0.03. In model_vs_data, the same numbers appear twice:

Model_vs_Data: r_work(re-computed) : 0.2030 r_free(re-computed) : 0.2639 ... After applying resolution and sigma cutoffs: n_refl_cutoff : 31257 r_work_cutoff : 0.2030 r_free_cutoff : 0.2639

But this totally contradicts what I told you earlier, sorry. I was assuming that they would be different.

I do have a general piece of advice, however: ignore the discrepancy, and just report the value that came out of refinement (because that is what will end up in the PDB). The difference in your case is relatively small, probably less than what you'd see if you calculated R-factors with (for instance Refmac), because of different implementations of bulk solvent correction and scaling, etc.*. (Even different versions of Phenix aren't guaranteed to yield identical R-factors, due to low-level changes.) Considering how difficult it can be to reproduce the statistics in published structures, a change of 0.0004 isn't enough to worry about.

-Nat

* http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2906258/?tool=pubmed _______________________________________________ phenixbb mailing list [email protected] http://phenix-online.org/mailman/listinfo/phenixbb

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