If you are refining against F's you have to find some way to avoid
calculating the square root of a negative number. That is why people
have historically rejected negative I's and why Truncate and cTruncate
were invented.
When refining against I, the calculation of (Iobs - Icalc)^2 couldn't
care less if Iobs happens to be negative.
As for why people still refine against F... When I was distributing
a refinement package it could refine against I but no one wanted to do
that. The "R values" ended up higher, but they were looking at R
values calculated from F's. Of course the F based R values are lower
when you refine against F's, that means nothing.
If we could get the PDB to report both the F and I based R values
for all models maybe we could get a start toward moving to intensity
refinement.
Dale Tronrud
On 06/20/2013 09:06 AM, Douglas Theobald wrote:
Just trying to understand the basic issues here. How could refining directly
against intensities solve the fundamental problem of negative intensity values?
On Jun 20, 2013, at 11:34 AM, Bernhard Rupp <hofkristall...@gmail.com> wrote:
As a maybe better alternative, we should (once again) consider to refine
against intensities (and I guess George Sheldrick would agree here).
I have a simple question - what exactly, short of some sort of historic inertia
(or memory lapse), is the reason NOT to refine against intensities?
Best, BR
