[QE-users] HOMO is higher than LUMO

[Lorenzo Paulatto]

Hello,
this happens when the system is a bad metal, with a pocket of valence electrons in a small Fermi surface. I.e., at some point in the Brillouin zone, the n-th band is lower in energy than the (n-1)-th band somewhere else, but these two bands do not touch. Just use a smearing for occupation and you'll solve the issue. Do not forget to recheck k-points convergence in the new case.

regards
--
Lorenzo Paulatto - Paris
On Feb 21 2021, at 7:05 pm, Mahmoud Payami Shabestari <mpayami at aeoi.org.ir> wrote:
> Dear QE-users/Developers,
>
> In DFT+U calculation of an AFM pure system, I used an input file in which I had not used "occupation" card, but only a few bands more than half of the number of electrons. Then, the SCF run was successful, and in the output I get both HOMO and LUMO values correctly aligned, i.e., E_LUMO > E_HOMO.
> However, when I add a metallic impurity (without changing the other parameters), the SCF run successfuly ends and in the output now I get E_HOMO > E_LUMO (!!).
> I do not know how to interpret this result. Is there any miscalculation?
> Any comments is highly appreciated.
>
> Best regards,
> Mahmoud Payami
> AEOI, Tehran, Iran
>
> Email: mpayami at aeoi.org.ir
> Phone: +98 (0)21 82066504
>
>
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