On Tue, 2011-10-11 at 10:47 -0700, Pavel Afonine wrote:
> better, but not always. What about say 80% or so complete dataset?
> Filling in 20% of Fcalc (or DFcalc or bin-averaged <Fobs> or else - it
> doesn't matter, since the phase will dominate anyway) will highly bias
> the map towards the model.
DFc, if properly calculated, is the maximum likelihood estimate of the
observed amplitude. I'd say that 0 is by far the worst possible
estimate, as Fobs are really never exactly zero. Not sure what the
situation would be when it's better to use Fo=0, perhaps if the model is
grossly incorrect? But in that case the completeness may be the least
of my worries.
Indeed, phases drive most of the model bias, not amplitudes. If model
is good and phases are good then the DFc will be a much better estimate
than zero. If model is bad and phases are bad then filling in missing
reflections will not increase bias too much. But replacing them with
zeros will introduce extra noise. In particular, the ice rings may mess
things up and cause ripples.
On a practical side, one can always compare the maps with and without
missing reflections.
--
After much deep and profound brain things inside my head,
I have decided to thank you for bringing peace to our home.
Julian, King of Lemurs
