Bart Hazes wrote:
I must admit that I've never understood the rationale for including dFc
for missing terms, although it has been discussed "lively" on a few
occassions. Yes, dFc is a better estimate for the true structure factor
than leaving out the term (equivalent to setting the amplitude to zero).
But dFc does not provide any information on how reliable the model is or
where the model may need to be changed. Since the latter is the main
function of electron density maps in my opinion I'm not confinced that
substituting dFc is a good idea at any time.
I believe fft does not by default do this- only if you use the "fillin"
keyword? One place where it might be important is in density
modification/ molecular averaging. Molecular averaging can be seen
as a numerical solution of the MR equations, finding a density map
which (A) obeys the NCS/intercrystal symmetry and (B) yields the
observed F's upon Fourier transformation. Now if on each cycle
you set the missing F's to zero, you are requiring it to have
as part of (B) zero amplitude for the missing reflections, which is
more restrictive and incorrect. If instead you allow the missing F's
to "float", calculating them on each cycle from the previous map
using the fillin option, someone has shown (don't have the
reference handy at the moment) that the F's tend toward the true F's
(in the case that they weren't really missing but omitted as part
of the test).
Ed
