[CP2K-user] [CP2K:22347] Re: Problems with cell optimization of graphene

[Riccardo Pezzetta]

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/eda89f78-adbd-4e89-b001-ad96e5165ed7n%40googlegroups.com.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.cp2k.org/archives/cp2k-user/attachments/20260729/231c2e6b/attachment.htm>