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I am able to convert cif files from Kunz et al., 1991 and Zolotarev et al., 2007 but I can’t convert the cif file of Cannillo et al.,1966. The warning was:
Error: This system is an alloy, but PWscf has no way of dealing with alloys.
Run again with –force-alloy (or –force) if you want to generate an (incomplete) output file anyway.I don’t understand why the system is defined as alloy …
We’ll come back to this in the feedback webinar, but let me tell it already here:
It’s about the _atom_site_occupancy variable. That variable tells which fraction of a position coordinate is occupied by the given element. For DFT input, it must be 1: either there is an atom at that coordinate, or there is no atom at all. In experimental cifs, however, you can have partial or mixed occupancy: an element occupying only, say, 37% of all availabe sites. Or two elements appearing at the same site, in a 29/71% ratio.
For the neptunite cif by Cannillo et al, you see this happening:
Fe2+1 0.34020 0.32110 0.09830 0.50000 0.01925
Ti1 0.34020 0.32110 0.09830 0.50000 0.01925
Fe2+2 0.08830 0.05610 0.11280 0.50000 0.01925
Ti2 0.08830 0.05610 0.11280 0.50000 0.01925Fe and Ti both share the same positions, with a 50/50 chance for each. That’s fine for an experimental cell, but DFT cannot model this (we’ll see a way around this later, using supercells).
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