[Wien] Fractional occupancies

[Stefaan Cottenier]

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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