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Dear Teresa,
when you speak about merohedrally twinned data, the intensities from the model
is calculated wrongly, as each intensity is the sum of two lattices. Because
refinement reduces the discrepancy between data and model, the difference will
cause overfitting, i.e. the parameters of the model will be (much) worse than
with including the twin law.
Best,
Tim
On Thursday, March 24, 2016 02:21:36 PM Teresa Swanson wrote:
> Thank you so much for your help. I thought that it would be incorrect to
> not use a twin law with heavily twinned data, although confirmation is
> good.
>
> Can someone explain to me why this (not using the twin law with twinned
> data) is such a big mistake? I'd like to be able to explain this to my
> colleagues, but am having a hard time wrapping my head around the technical
> reasons why. Is it because you're processing essentially two datasets with
> inconsistent indexing?
>
> On Tue, Mar 22, 2016 at 11:43 AM, Keller, Jacob <[log in to unmask]>
>
> wrote:
> > >Is it technically/theoretically incorrect to *not* use a twin law for a
> >
> > heavily twinned structure? Is there any reason to think those maps may
> > produce different results as refining with a twin law? Or is either
> > refinement option theoretically acceptable?
> >
> > Yes, it is incorrect because you are discarding a **very** important
> > piece of information about your data, similar to assigning the wrong space
> > group, or better still, like merging two datasets with inconsistent
> > indexing.
> >
> > JPK
- --
- --
Paul Scherrer Institut
Tim Gruene
- - persoenlich -
OFLC/102
CH-5232 Villigen PSI
phone: +41 (0)56 310 5297
GPG Key ID = A46BEE1A
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