Dear all,
I am currently switching from Quantum ESPRESSO (QE) to CP2K as my main computational software package (currently using version 8.2) and I am struggling a bit with the convergence tests for the PW cutoffs (REL_CUTOFF and CUTOFF). My intended structures to investigate are moderate-sized organic molecules adsorbed onto a (101) surface of anatase (TiO2) and as such I am using an optimized structure from my QE calculations as a representative system to do my convergence tests on (see attached file 3mppa-md1.inp). I based my method on the method proposed by Prof. Hütter in a previous post, whereby I first set REL_CUTOFF=CUTOFF and increase the value until I reach convergence. The results I got are the following:
# Grid cutoff vs total energy
# Date: Fri Jun 2 14:09:32 CEST 2023
# PWD: /home/lluntadilal/cp2k/scf/dft/test/final/convergence/cutoff
# Cutoff (Ry) | Relative cutoff (Ry) | Total energy (a.u.) | Total charge (a.u.)
300 300 -3303.4693919783 0.0000049144
400 400 -3303.4678396250 -0.0000000118
500 500 -3303.4658109937 -0.0000000003
600 600 -3303.4675587607 -0.0000000003
700 700 -3303.4671967038 -0.0000000003
800 800 -3303.4663947747 -0.0000000003
900 900 -3303.4659684557 -0.0000000003
1000 1000 -3303.4657584430 -0.0000000002
1100 1100 -3303.4651289391 -0.0000000002
1200 1200 -3303.4650897922 -0.0000000003
1300 1300 -3303.4651986119 -0.0000000002
1400 1400 -3303.4651800964 -0.0000000002
1500 1500 -3303.4650037920 -0.0000000002
1600 1600 -3303.4650956961 -0.0000000002
1700 1700 -3303.4651701336 -0.0000000002
1800 1800 -3303.4651578599 -0.0000000002
1900 1900 -3303.4651999129 -0.0000000002
2000 2000 -3303.4651936486 -0.0000000002
I have several questions about this:
Any help would be greatly appreciated!
Kind regards,
Léon
P.S. For those looking at my structure in a graphical environment and noticing that some atoms are outside the unit cell. This is due to the fact that my cell was monoclinic and I made the unit cell orthorhombic to save computational time and trouble (some codes have trouble dealing with cells that are not orthorhombic), but I did not yet wrap all my atoms into the unit cell. However, I don't think this should be a problem under PBC.
I had the same exact issues with the convergence !! exactly the same. I was also confused about the largest exponent as well. When I used this https://www.cp2k.org/faq:cutoff, I got a very high cutoff value which when I employed deviated very much from the experimental values. So may be you need to look at other values, other than the total energies. The total energies we get here are not by any mean realistic. When I compared the total energy values I got from VASP with the values we get from CP2K (for single elements), I got completely different results. So when I increased the basis sets I was able to get the same values for Al and Nb, other elements like Ti REALLY need optimization of the basis sets and pseudopotentials. Therefore you should try to look at other values other than the total energies (like the lattice parameters for example) to be able to deduce a final conclusion about the cutoff value.
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