[buster-discuss] two questions : one about negative reflections and one about Simulated annealing

[Ian Tickle]

Hi Nicolas

As far as I know, Buster doesn't have a Simulated Annealing option, but
maybe the Buster developers will have more to say on that.

On your second question, there's an important step between getting the
merged intensities and performing the refinement with Buster.  Again as far
as I know Buster only uses amplitudes (Fs) as input.  Fs unlike
intensiities cannot be non-negative by definition.  So we need to convert
all the merged intensities including negative ones to positive Fs.  This is
usually done by French & Wilson Bayesian estimation:

https://warwick.ac.uk/fac/sci/statistics/staff/academic-research/french/publications/bayes_impact_french_wilson.pdf

Also see the reference to the original 1978 paper cited there.

This algorithm, or a modification of it, is implemented, among others, in
the Truncate, CTruncate and StarAniso programs.  For example, StarAniso
computes the Fs using a very accurate numerical method instead of using
table look-ups, and takes into account the effect of anisotropy on the
Bayesian prior.

Best wishes

-- Ian


On Tue, 22 Jun 2021 at 10:29, Nicolas Foos <nicolas.foos at ibs.fr> wrote:

> Dear buster users and developers,
>
> I have two questions : one simple : is it possible to perform a simulated
> annealing using buster. I didn't see anything like that in the manual, but
> maybe I miss it.
>
> The second question : how autoBuster handle the negative reflections
> present in the data file ?
> - Does it ignore them ?
> - Does it fill them ?
> - Does it set them to zero ?
> What's you opinion about that point, should we use them or not. I would be
> glad to have your opinion about that point.
>
> To let you know a bit more : I am working on time resolve project. The
> Data were originally collected in XFEL, then processed using CrystFel. And
> then based on Fo(state1)-Fo(state2) difference map calculation, I did an
> extrapolation to refine the model. The data which generates my questions
> are the Extrapolated ones.
>
> I am fully open to any comments.
>
> All the best,
>
> Nicolas Foos
>
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