**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 !**