Hi Ed,
I am not sure I understand this - perhaps we are talking about
different
things. Even if by inversion procedure you mean simple calculation
of
(2fo-fc)*exp(i*phi), the fc is still technically a product of the
refinement, which unless based on trivial least square target (i.e.
no
weights) does factor in experimental errors. The (2mFo-DFc) map is
even
more obviously dependent on the errors. Again, I believe that the
differences will be minor, but if one calculates a map with refmac
either with or without factoring in experimental errors, there will
be
*some difference*. Thus, the experimental errors will affect the
resulting map. Could you please clarify?
Yes, we are talking about different things. I refer to the case that we
have an amplitude term with its uncertainty (no matter whether it is Fo
or
Fo^2 or Fo-Fc or 2mFo-DFc or ...) plus a phase with its uncertainty. In
normal
everyday applications we use FFT which ignores (i) the uncertainties of
both terms, (ii) the missing data. By doing an FFT we produce a map
which
exactly reproduces the input data (even if they are missing data which
are
reproduced with an amplitude of zero). What I have been saying is that
in
the presence of uncertainties and missing information the data do not
define a single map, but a whole set of maps which are statistically
consistent with the data and the question then arises : 'which map
should
I be looking at ?'. I happen to mention the maximum entropy method as a
possible solution to this problem.
I think that we are not comparing ML to no-ML (or maximum entropy),
but
rather ML inflated by experimental errors vs pure ML that ignores
them.
I may be crazy or stupid (or both), but certainly not crazy/stupid
enough to "doubt the importance of maximum likelihood for
refinement".
(On the other hand, one who promises to never doubt maximum
likelihood
shall never use SHELX :)
We definitely talk about different things. My arguments had nothing to
do
with treatment of errors in refinement. The question I was tackling was
how you
go from |F|,sig(|F|),phase to a map in the presence of errors and
missing
data.
Nicholas
--
Nicholas M. Glykos, Department of Molecular Biology
and Genetics, Democritus University of Thrace, University Campus,
Dragana, 68100 Alexandroupolis, Greece, Tel/Fax (office)
+302551030620,
Ext.77620, Tel (lab) +302551030615, http://utopia.duth.gr/~glykos/
