[Wien] Fractional occupancies

[Rocquefelte]

Dear all,

I am not sure that the initial question of Dean Keeble was about 
"partial occupancy" of a site.
In fact the first mail use an ambiguous terminology "fractional 
occupancies". See the text below:

Uploading a .cif file and typing the the co-ordinates by hand yield the 
same problem - the fractional occupancies in my material are not 
correctly inputted. Examining the .struct file doesn't seem to give any 
clues either. Can (and if so, how) do I specify fractional occupancies?

Such terminology is not very clear, because we should speak about 
"fractional coordinates" or "partial occupancies", but "fractional 
occupancies" has no real meaning.

So, what do you really want Dean? If you wanted to know how to handle 
with "partial occupancies" the answer of Stefaan is perfect.
If, you wants to deals with "fractional coordinates" the answer is 
different, and your question must be understand in another way.

If all your positions are fully occupies and if you have transformed 
your cif file in struct file using cif2struct, you must also check if 
the space group setting you have is supported by Wien2k (particularly 
for low symmetry systems). If not, you should transform your struct file 
in such a way to respect the Wien2k space group setting.

I hope that this mail will be helpfull. Sorry for such comment if the 
initial question was really about "partial occupancies".


Regards

Xavier





Stefaan Cottenier a écrit :
> Pretty hopeless, I'm afraid. Wien cannot deal with fractional 
> occupancies, it needs real atoms at every site. A correct solution that 
> would need way too much calculations is to take a rather large 
> supercell, and to make all possible configurations of atoms that are 
> allowed within your constraints. Then average your EFG over all these 
> possibilities. For sure your supercell will need to be too large and the 
> number of configurations accordingly immense to be of practical use.
>
> There is a possibility to make the 'virtual crystal approximation' by 
> giving fractional Z-values as input, but its accuracy is doubtfull (even 
> more for EFG's). See the mailing list archive.
>
> We have been put for a similar problem some time ago (see PRB 74 (2006) 
> 184108), and found it insoluable for EFG's. For Mossbauer isomer shifts, 
> we could deal with it, however.
>
> Stefaan
>
>   
>> Dear all,
>>
>> I am a relatively inexperienced user of Wien (as my question is sure to 
>> imply more aptly than I realise).
>>
>> I'm attempting to find the EFG of various ferroelectric ceramics and I 
>> can't seem to enter the structure correctly.
>> Uploading a .cif file and typing the the co-ordinates by hand yield the 
>> same problem - the fractional occupancies in my material are not 
>> correctly inputted. Examining the .struct file doesn't seem to give any 
>> clues either. Can (and if so, how) do I specify fractional occupancies?
>>
>> Thankyou in advance for your swift and benignant reply,
>>
>> Dean Keeble
>> Dept. of Physics
>> Warwick University
>> UK
>>
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>>
>>  
>>
>>     
>
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