[phenixbb] using Fobs=0 (instead of "changing TLS groups")

Alexandre Urzhumtsev sacha at igbmc.fr
Tue May 18 00:32:20 PDT 2010

Dear all,

sorry to interfere late and also to update the subject (that had nothing to 
"TLS groups" in the last messages).
	I would like also to point out an article in which in fact Pavel is one of 
the co-authors :
		Lunin et al (2002), Acta Cryst, A58, 270-282
and that is directly relevant to the question of using Fobs=0.

	Comparison of maximum-likelihood-based and least-squares refinements and a 
development of the ML-target into a Taylor series show that the 
ML-refinement has two basic features :

1) it defines in a very special way the weights for some terms

2) for weak reflections, especially when the model is incomplete (that is 
always the case), it suggests fitting Fcalc to 0 and not at all to the 
measured Fobs.

You may look at the corresponding plots in the article.
	This means that, complementarily to the French & Wilson study of 
experimental data, an option to use Fobs=0 maybe not so meaningless in a 
refinement program especially if by some reasons one wants to use 
LS-refinement and not ML.

Best regards,


PS : My personal request to the phenixbb users - please take care and put a 
right subject when posting your original message

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