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different occopts give different qualitative result

PostPosted: Wed Jan 23, 2019 10:44 am
by ryne60
Dear all,

I've been calculating a system with two slightly different structures, say structures A & B. The first set of calculations using occopt 6 and resulted in the total energy of A (EA) smaller than the total energy of B (EB). However, when I use occopt 7 the results show that EB < EA.

Has anyone experienced this before? Can anyone tell me which occopt is better to use?

Thanks!

Re: different occopts give different qualitative result

PostPosted: Fri Jan 25, 2019 1:19 pm
by ebousquet
Dear ryne60,
The different occopt values use a different "mathematical" expression to smear the electrons and can thus end into different results for the same tsmear value. What you can do is to check how your system change by varying the tsmear value, this would be more consistent. By using occopt=3 (Fermi-Dirac), you at least know that the tsmear value corresponds to a "physical" temperature (temperature of the electrons) while it does not really has a physical meaning for the other cases.
Best wishes,
Eric