[CP2K-user] [CP2K:22346] Re: Problems with cell optimization of graphene

[Augustin Bussy]

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/9a83c0bb-325b-4532-bd27-02705f3932a3n%40googlegroups.com.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.cp2k.org/archives/cp2k-user/attachments/20260729/a2f887de/attachment.htm>