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CCP4BB  February 2010

CCP4BB February 2010

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Subject:

Re: What is an aceptable spread in ADP values?

From:

Ethan Merritt <[log in to unmask]>

Reply-To:

Ethan Merritt <[log in to unmask]>

Date:

Wed, 3 Feb 2010 10:38:10 -0800

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On Wednesday 03 February 2010 10:04:36 Pavel Afonine wrote:
 I always suggest to run these refinement jobs and see which 
> one gives the best result:
> 
> 1) TLS + individual isotropic ADP refinement (tls+individual_adp);
> 2) TLS + group individual ADP refinement (tls+group_adp);

I do not think that your (2) is a physically plausible model.
I would dis-recommend this option.

> 3) individual isotropic ADP refinement;
> 4) group individual ADP refinement (with one refinable B per residue);
> 5) group individual ADP refinement (with two refinable B per residue).

I also tend to agree with Ed that option (5) has little physical
plausibility in its favour.  Better to stick with individual Biso
and tight B restraints.

	cheers,

		Ethan

> + on top of it you can add automatic weights optimization to remove the 
> arbitrariness in what is "loose" and "tights" restraints.
> 
> This will give the conclusive, rock solid answer about which ADP 
> parameterization and refinement protocol is good for given model and 
> data. An alternative is an endless speculation.
> 
> In the future the proper parameterization will be chosen automatically 
> based on a large array of data, model and map characteristics.
> 
> >  The existing
> > two-adp-groups-per-residue implementation (CNS and phenix) is, imho, an
> > example of *improper* parametrization.  
> 
> As you see, in phenix.refine you can combine any B-factor refinement 
> strategy with any (group, individual iso, aniso, tls), and apply it to 
> any selected part of your structure. So, I assume at this point of the 
> software automation, it is up to a smart researcher to decide which 
> refinement strategy to use. You cannot blame the software for giving you 
> the freedom to do what you may want to do. For example, if you choose to 
> refine two-B-factors per residue when you could safely refine individual 
> B-factors, it will be the example of improper parameterization that you 
> have chosen (and not the software did for you). In phenix.refine 
> technically you can refine individual isotropic or anisotropic B-factors 
> at any resolution - the program will not crash, and it will be up to the 
> user whether he/she will enjoy the results. Like I said, in the future 
> the model parameterization will be done automatically.
> 
> I guess I'm going off this discussion - otherwise phenix.refine will get 
> less new options in the future if I keep writing -:)
> 
> All the best!
> Pavel.
> 
> 



-- 
Ethan A Merritt
Biomolecular Structure Center
University of Washington, Seattle 98195-7742

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