Surely the "best" model is the one that the referees for your paper
are happy with?
I have found referees to impose seemingly random and arbitrary
standards that sometime require a lot of effort to comply with but
result in little to no impact on the biology being described. Mind you
discussions on this email list can be a useful resource for telling
referee's why you don't think you should comply with their "rule of
thumb".
Simon
On 27 Oct 2010, at 20:11, Bernhard Rupp (Hofkristallrat a.D.) wrote:
Dear Young and Impressionable readers:
I second-guess here that Robbie's intent - after re-refining many
many PDB
structures, seeing dreadful things, and becoming a hardened cynic -
is to
provoke more discussion in order to put in perspective - if not
debunk-
almost all of these rules.
So it may be better to pretend you have never heard of these rules.
Your
crystallographic life might be a happier and less biased one.
If you follow this simple procedure (not a rule)
The model that fits the primary evidence (minimally biased electron
density)
best and is at the same time physically meaningful, is the best
model, i.
e., all plausibly accountable electron density (and not more) is
modeled.
This process of course does require a little work (like looking
through all
of the model, not just the interesting parts, and thinking what
makes sense)
but may lead to additional and unexpected insights. And in almost
all cases,
you will get a model with plausible statistics, without any reliance
on
rules.
For some decisions regarding global parameterizations you have to
apply more
sophisticated test such as Ethan pointed out (HR tests) or Ian uses
(LL-tests). And once you know how to do that, you do not need any
rules of
thumb anyhow.
So I opt for a formal burial of these rules of thumb and a toast to
evidence
and plausibility.
And, as Gerard B said in other words so nicely:
Si tacuisses, philosophus mansisses.
BR
-----Original Message-----
From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf
Of Robbie
Joosten
Sent: Tuesday, October 26, 2010 10:29 PM
To: CCP4BB@JISCMAIL.AC.UK
Subject: [ccp4bb] Rules of thumb (was diverging Rcryst and Rfree)
Dear Anthony,
That is an excellent question! I believe there are quite a lot of
'rules of
thumb' going around. Some of them seem to lead to very dogmatic
thinking and
have caused (refereeing) trouble for good structures and lack of
trouble for
bad structures. A lot of them were discussed at the CCP4BB so it may
be nice
to try to list them all.
Rule 1: If Rwork < 20%, you are done.
Rule 2: If R-free - Rwork > 5%, your structure is wrong.
Rule 3: At resolution X, the bond length rmsd should be < than Y
(What is
the rmsd thing people keep talking about?) Rule 4: If your
resolution is
lower than X, you should not use_anisotropic_Bs/riding_hydrogens
Rule 5: You should not build waters/alternates at resolutions lower
than X
Rule 6: You should do the final refinement with ALL reflections Rule
7: No
one cares about getting the carbohydrates right
Obviously, this list is not complete. I may also have overstated
some of the
rules to get the discussion going. Any addidtions are welcome.
Cheers,
Robbie Joosten
Netherlands Cancer Institute
Apologies if I have missed a recent relevant thread, but are lists of
rules of thumb for model building and refinement?
Anthony
Anthony Duff Telephone: 02 9717 3493 Mob: 043 189 1076
=
