What is exactly
the Rietveld decomposition formula used for obtaining Fobs, allowing the R_Bragg calculation ?
 
 
 
 

A partition is made according
to the calculated structure factors.
 

"|Fobs|" are thus more or less biased |F|,
due to overlapping.
 
 

For instance :
if the Fcalc of two exactly overlapping reflections are equal, then the Fobs will be equal too.
If the two Fcalc are different, the ratio of the two Fobs will equal the ratio of the two Fcalc.
 
 

Find another way to proceed if you can !
 
 

Anyway, iterating this formula starting from arbitrarily identical Fcalc provides a way to estimate Fobs without any starting structure model.
 
 
 

And what about ESDs ?
 
 

We know how to obtain ESDs for |F|
(single crystal data)

But how to obtain ESDs for "|Fobs|" ?

In fact, we obtain "ESDs".....
 
 
 

But who needs really ESDs
at the solving structure stage ?
 

In SHELXS, ESDs allow to omit dubious weak |F|,
and this is their only interest,
but it is recommended to use
this possibility with care !