[CP2K-user] [CP2K:22362] Re: Problems with cell optimization of graphene

[Frederick Stein]

Dear Riccardo,
You are using smearing. Can you turn it off? Are all orbitals fully 
occupied? How does the band-gap change with cell volume? Beware that our 
RPA-implementation does NOT consider partial occupation. Even worse, 
partially occupied orbitals are considered as fully occupied by our 
implementation such that the energies may even differ with the available 
orbitals.
In addition, I am not entirely sure how well the 2D-periodicity is covered 
in your executable of CP2K and I would rather use PERIODIC XYZ instead.
Best,
Frederick

Riccardo Pezzetta schrieb am Mittwoch, 29. Juli 2026 um 14:12:13 UTC+2:

> Thank you very much for the answer.
> Kind regards,
>
> Riccardo Pezzetta 
>
> Il giorno mercoledì 29 luglio 2026 alle 11:15:38 UTC+2 Augustin Bussy ha 
> scritto:
>
>> Dear Riccardo,
>>
>> your HFX input is not ideal, because a 4 Angstrom cutoff is probably too 
>> short. There are 2 things to consider when selecting a value for the cutoff:
>> 1) is should not exceed half the cell size (OK in your case)
>> 2) it should be long enough. In most cases, a value of ~6 Ang is deemed 
>> sufficient, but technically, the ideal cutoff depends on the system band 
>> gap (for smaller gap systems, a longer cutoff might be required)
>>
>> I suggest you take a supercell, and extend the cutoff.
>>
>> Note that the value of EPS_PGF_ORB you use is way too small (1.0E-16), 
>> and it will dramatically impact performance. Please read section 2.2 of 
>> https://pubs.acs.org/jpcbfk/article/130/4/1237/5088250/The-CP2K-Program-Package-Made-Simple 
>> for an explanation.
>>
>> If using a supercell makes the RPA part of your calculation too expensive 
>> (RPA scales as N^4 with system size), consider using the low-scaling 
>> implementation; it is particularly efficient for 2D systems.
>>
>> Best,
>> Augustin
>>
>> On Wednesday, 22 July 2026 at 14:59:37 UTC+2 Riccardo Pezzetta wrote:
>>
>>> Dear CP2K community,
>>> I am trying to optimize the lattice constant of a graphene cell with RPA 
>>> at the Gamma point. I performed energy calculations with different lattice 
>>> constants, but I get a very strange behavior close to the minimum. As shown 
>>> in the image attached to this mail, the RPA energy shows an unphysical drop 
>>> close to the minimum. With PBE the curve is regular, while the HF at PBE 
>>> energy (i.e., RPA without the correlation term) shows the same irregular 
>>> profile as RPA, so the issue seems to originate from the Hartree-Fock 
>>> exchange term. In my input, that you find attached to the mail, the only 
>>> parameter I set in the HF section is the cutoff of the truncated Coulomb 
>>> operator, I set it equal to 4 Angstrom so that it does not exceed half the 
>>> cell size for all values of the lattice constant considered. Have you ever 
>>> encountered a similar problem? What can be the cause of this unphysical 
>>> behavior? Thank you in advance for your answer.
>>> Kind regards,
>>>
>>> Riccardo Pezzetta
>>>
>>

-- 
You received this message because you are subscribed to the Google Groups "cp2k" group.
To unsubscribe from this group and stop receiving emails from it, send an email to cp2k+unsubscribe at googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/cp2k/0bf0cd2a-8561-4050-b4c0-513dc78d4c78n%40googlegroups.com.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.cp2k.org/archives/cp2k-user/attachments/20260802/8150c5a9/attachment.htm>