Problems with cell optimization of graphene

[Riccardo Pezzetta]

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@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