GW calculation on methanol

Concerns issues with computing quasiparticle corrections to the DFT eigenvalues - i.e., the self-energy within the GW approximation (-g n), or considering the Hartree-Fock exchange only (-x)

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subhayan
Posts: 32
Joined: Thu Sep 26, 2019 7:48 am

GW calculation on methanol

Post by subhayan » Fri Dec 31, 2021 2:32 pm

Dear developer,

I am trying to run a GW calculation on an isolated methanol molecule. I seem to be getting an extremely high (LUMO-HOMO) gap of more than 10 eV, against a PBE gap of ~5.5 eV. I suspect that my calculation might be underconverged with respect to some parameters but I'm not really sure what. I am attaching the input and the output of two separate calculations with different values of RandGvec , CUTRadius and NGsBlkXp. The QP gap does not seem to change much between the two calculations. Could you kindly take a look and let me know what you think?

Best regards,
Subhayan
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Subhayan Roychoudhury
Postdoctoral researcher
Lawrence Berkeley National Laboratory

subhayan
Posts: 32
Joined: Thu Sep 26, 2019 7:48 am

Re: GW calculation on methanol

Post by subhayan » Sat Jan 01, 2022 5:15 am

Hi,

A quick update on this. I reran the calculation with a much higher number of (600) bands but unfortunately the gap doesn't seem to change much :cry:
Any suggestion will be extremely appreciated.

Best regards,
Subhayan
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Subhayan Roychoudhury
Postdoctoral researcher
Lawrence Berkeley National Laboratory

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Daniele Varsano
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Re: GW calculation on methanol

Post by Daniele Varsano » Sun Jan 02, 2022 6:49 pm

Dear Subhayan,
the value you get seems reasonable, please have a look at the GW100 paper (J. Chem. Theory Comput. 2015, 11, 5665−5687) where a gap of about 10.7 eV is reported for GW at Plasmon Pole level.

In general converging molecules is rather hard in terms of unoccupied bands and cutoff for the dielectric matrix.
Comment on your inputs:
* RandQpts this is the number of q vectors used for a Monte Carlo integration, set it to 1 million or more otherwise it could be inaccurate (large statistical error)
* CUTRadius you can keep it fix to half of your supercell
* GTermKind="BG" can help you in accelerating the convergencence wrt the bands in the GW summation.

Best,
Daniele
Dr. Daniele Varsano
S3-CNR Institute of Nanoscience and MaX Center, Italy
MaX - Materials design at the Exascale
http://www.nano.cnr.it
http://www.max-centre.eu/

subhayan
Posts: 32
Joined: Thu Sep 26, 2019 7:48 am

Re: GW calculation on methanol

Post by subhayan » Mon Jan 03, 2022 9:02 am

Hi Daniele,

Thanks a lot for the reference and the clarification :-)

Best regards,
Subhayan
Subhayan Roychoudhury
Postdoctoral researcher
Lawrence Berkeley National Laboratory

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