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Re: [sdpd] Re: Restraints and so on
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<p><br>
> >we can solve structures much more complex than those we can refine <br>
> without restraints.<br>
><br>
>If a minimum of a cost function is found for a constrained structure <br>
>model in direct space, but the unconstrained refinement is <br>
>impossible, one does not have criteria (e.g. distances, angles, <br>
>their e.s.u. etc.) to assert that the structure is determined or <br>
>soled. For such cases a special term should be proposed instead of <br>
>structure solution, say, structure optimization.<br>
<br>
Hum, this eliminates almost all proteins, none was determined or <br>
solved, even from<br>
single crystal data, according to your criteria. It will be hard to <br>
define an exact limit<br>
since all determined crystal structures are more or less wrong, <br>
depending on the<br>
degree of accuracy you want to impose, some parts of the structures being less<br>
accurate than others.<br>
<br>
Remember that in the sixties and before, the H atoms were frequently not<br>
located from single crystal data. Even nowadays, people use the structure<br>
solution software (SHELXL, etc) suggestions for obtaining the supposedly<br>
riding hydrogen atoms which coordinates are then "refined" with restraints.<br>
We have no estimation of the percentage of false H atoms which were<br>
"seen". Not speaking of the multitude of structures from single crystal data<br>
that cannot be refined without restraints but are certainly "correct enough".<br>
<br>
So, "structure optimization" should be reserved to more noble works<br>
I think, when there is really an optimization (suggesting a better <br>
final result).<br>
Because if you optimize a wrong model, it will continue probably to be wrong,<br>
which is not the sense I would expect for an optimization. Indeed, quantum<br>
optimization is supposed to say if the model is wrong (hum...).<br>
<br>
Finally, as a conclusion, we can be sure that, with SDPD, we have extended<br>
the domain of the "uncorrect enough" structures... giving them a chance<br>
to be really optimized with sophisticated quantum chemical software, though<br>
probably, false proposals will be in such results as well. Science progressing<br>
(too much ?)...<br>
<br>
This suggests also that, as for papers in Acta Cryst where the Fo/Fc are<br>
used for the CheckCIF process, powder works should provide the<br>
raw powder diffraction pattern, giving a chance to the referee to make a<br>
better job - hum again... Any special PLATON version for powder works ?<br>
I think that very few will pass the tests without a huge list of ALERT A.<br>
<br>
Best,<br>
<br>
Armel<br>
<br>
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