Yuri,
Detwinning relies on having both twin-related reflections present to
calculate either/both of the the de-twinned data values. Therefore it
magnifies incompleteness depending on where your missing data is with
respect to the twin operator.
I'd recommend against trying to do this with a twin fraction close to
0.5. From the DETWIN docs:
Itrue(h1) = ((1-tf)*iTw(h1) -tf*iTw(h2)) / (1-2tf)
i.e. tf = twin fraction, so 1/(1-2tf) becomes a large number and it's
multiplying a weighted term of the form: (iTw(h1) - iTw(h2)) which
becomes a very small number as the twin fraction approaches 0.5. The
latter difference can easily be less than sigma(I), and so the
signal/noise of your data plummets.
Better to use REFMAC and phenix.refine's abilities to compensate for the
twin fraction directly in refinement and leave your data as it is.
Phil Jeffrey
Princeton
On 9/29/11 10:03 AM, Yuri Pompeu wrote:
After I ran DETWIN with the estimated 0.46 alpha, my completeness for the
detwinned data is now down to 54%!!!
Is this normal behavior? (I am guessing yes since the lower symmetry untwinned
dat is P1 21 1)
