[CP2K-user] [CP2K:22334] 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 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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