[CP2K-user] [CP2K:22397] Re: Problems with cell optimization of graphene

[Riccardo Pezzetta]

Dear Professor Stein,

Thank you very much for your reply.

I checked the convergence with respect to the smearing temperature. I found 
that the energy is very similar for electronic temperatures of 100, 200, 
and 300 K, while a noticeable change starts to appear at 500 K. However, if 
I completely remove the smearing, the PBE SCF calculation does not converge.

I also checked the convergence with respect to the size of the cell along 
the z direction.

Following your suggestion, I added the following section to print the 
orbital occupations:

&PRINT
&MO
OCCUPATION_NUMBERS
&END MO
&END PRINT

I found that the occupation numbers are either 2 or 0 for the different 
lattice constants considered, with no partially occupied orbitals.

Finally, I tried to perform a pure HF calculation to investigate whether 
the problem originates entirely from the Hartree-Fock exchange term. 
However, the HF calculation does not converge, even when using a relatively 
high electronic temperature or when mixing HF exchange with PBE exchange or 
when using PERIODIC XYZ instead of PERIODIC XY.

Do you have any suggestions on what I could try next?

Thank you again for your help.

Best regards,

Riccardo Pezzetta

Il giorno domenica 2 agosto 2026 alle 17:26:34 UTC+2 Frederick Stein ha 
scritto:

> 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
>>>>
>>>

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