[QE-users] occupation in biological molecules and band gap

[xavier neto]

Dear Mahmoud Payami,

Thank you for your answer. It helps me a lot.

So, I can use the Fixed occupation and, if the energy levels show a 
small bandgap, I introduce the Smearing. Is it right?


About the second question. I've read that the result of HOMO - LUMO, 
obtained after the SCF calculation, can give the bandgap. Can I obtain 
the bandgap in that way? If yes, I just have to add the nbnd function in 
the input file?

On 11/1/21 02:04, Mahmoud Payami Shabestari wrote:
> Dear José Xavier,
> Hi.
> Smearing for the occupations is used when the levels near HOMO are so 
> close (commonly in metals near Fermi level) that in consecutive 
> iterations they may exchange positions and lead to nonconvergence. For 
> semiconductors with 3-5 eV gap I think it won't help.
> Bests
> Mahmoud Payami
> NSTRI, AEOI, Tehran, Iran
>
>     From: José Xavier via users <users at lists.quantum-espresso.org>
>     To: "users at lists.quantum-espresso.org"
>     <users at lists.quantum-espresso.org>
>     Date: Mon, 1 Nov 2021 00:18:55 +0000 (UTC)
>     Subject: [QE-users] occupation in biological molecules and band gap
>     Dear all,
>
>     I'm new in QE. The group that I'm working with usually calculates
>     the properties of biologically relevant molecules, like amino
>     acids, hormones, neurotransmitters, and drugs, and I would like to
>     introduce the QE code to the group. Their previous papers have
>     shown that the bandgap of these crystals is something between 3-5
>     eV, which I understand to be in the semiconductor range.
>
>     I've watched/read some tutorials about QE, and some of these
>     showed that the occupation of semiconductors should be "smearing",
>     other ones that it should be "fixed", but no one showed
>     calculations with this kind of molecules. What value should I use
>     for the SCF and electronic calculations of the crystals that I'm
>     going to work on (amino acids, hormones, neurotransmitters, and
>     drugs)?
>
>     *Are there any differences when the molecule has a metal, like the
>     Heme group?
>
>     Besides, I would like to ask if there is a calculation that I
>     could perform to obtain the bandgap of these molecules. Is it only
>     possible if I create the band structure figure and calculate the
>     difference between the peaks in VB and CB?
>
>     Thank you for your help,
>
>     Sincerely,
>     José Xavier
>
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