[Wien] Structure Factors and unit cell content

[Semen Gorfman]

Hi Georg!

Sorry, I just thought that you meant B-centered cell [1/2, 0, 1/2].
For the case of a body centered one the extinction rules should be
exactly as you wrote it. 

Which lattice parameters do you have in your case.struct file? Are
they really "orthorhombic"? It might be that you defined the primitive
unit cell, which is related to the orthorhombic cell as [1,0,0],
[0,1,0], [1/2,1/2,1/2]. It does not look to be the case since you told
that extinction rules are fulfilled in lapw3 output file, but
anyway...
There are very often some unclear things happen in WIEN2k "symmetry"
block when it comes to the definition non-primitive cells like body
centered or face-cenetred.

Best regards
Semen


> Hi again !

>> For example you can use the data which you have already calculated
>> with lapw3. If you want to transform them into the structure factors
>> for the complete b-centered cell you should simply multiply the
>> obtained values by 2 for the case h+l=2n and by 0 when h+l=2n+1.

> Is this really that simple in the case of a body-centered cell ? The 
> orientation of the primitive cell is different from that of the original 
> body-centered cell, so I would think that the HKL Indices do not match each 
> other. 
> Apart from that I have tried to refine the structure with the calculated HKL 
> and if there is only a constant factor which has to be multiplied for all HKL 
> this should be covered by the refinement of the scaling factor - which is not 
> the case.

> Regarding your note on the extinction rules: The cell is body-centered, so I 
> would have the reflection-condition h+k+l=2n. This is already taken care of 
> by lapw3 as it seems, because there are only such hkl in the output-list. 

> regards

> Georg



>> > Dear WIEN users,
>> >
>> > I am trying to calculate structure factors for a carbide contaning Sc and
>> > Fe. The spacegroup of the compound is Immm (No. 71), i.e. I have a
>> > orthorhomic, b-centered cell. There are 1 Fe, 2 Sc and 4 C Atoms in the
>> > asymmetric unit, the atoms are located at the special positions 2b(Fe),
>> > 2d(Sc1), 4i(sc2) and 8l(C). The complete cell-content is 2Fe, 6Sc and 8C.
>> >
>> > The calculations converged without problems and the structure looks fine
>> > when I plot it using xcrysden.
>> >
>> > Then I tried to calculate a HKL File using lapw3. In the output I obtain
>> > an F000 value of 113 which is the sum of 3xSc, 1xFe and 4xC. This is the
>> > content of the reduced primitive, not body-centered cell.
>> >
>> > Is there a way to make lapw3 use the complete b-centered cell for the
>> > calculation of the Intensities so they are directly comparable with
>> > experimental data ?
>> >
>> > Thank you for any help !
>> >
>> > Georg
>> >
>> >
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>>
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