Dear Stefaan, Thank you for the answer.
I am using the PBE-GGA functional. According to the http://prb.aps.org/abstract/PRB/v79/i8/e085104 paper, it is indeed the best choice for Fe. However, LDA seems to be the best for Ge. Well, it looks like that the Fe-Ge system is not very convenient for this kind of optimizations. Best regards, Volodymyr Le 15/07/2011 15:19, Stefaan Cottenier a ?crit : > >> I am working on a Fe-Ge compound with about 17% vacancies on one site. I >> have set this site as a fully occupied and I am currently trying to >> optimize the volume and the atomic coordinates. >> >> For the initial volume optimization run, I have reduced RMTs by 5%, and >> as a result I got an the "optimized" volume smaller than the original. I >> believe this is expected due to the reduced RMTs. > > No, this is not related to the choice of RMT. RMT is a mathematical > construct only. Whatever RMT you choose (within reasonable limits), > you should get the same observable properties (e.g. the volume, in > your case) if you perform well-converged calculations. > > First estimate what is the minimal volume that you will ever want to > calculate for this compound, and then take the RMTs such that they'll > not overlap for this minimal volume. > >> However, in reality, >> it would be expected to get a slightly larger volume for this new >> "stochiometric" phase. > > Which functional are you using, and how large is 'slightly'? If you > use e.g. LDA, it will usually underestimate the volume by several > percent, making the volume for the stochiometric compound perhaps > smaller than the experimental volume of the off-stochiometric compound. > > You might have a look at > http://prb.aps.org/abstract/PRB/v79/i8/e085104 to get an idea about > which functional is likely to provide the optimal volume for the > elements you are using. > > Stefaan > _______________________________________________ > Wien mailing list > Wien at zeus.theochem.tuwien.ac.at > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
