[CP2K-user] [CP2K:18882] Problems with converging PW cutoff

[Léon Luntadila Lufungula]

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 <https://groups.google.com/g/cp2k/c/ySUAYEgwmhc>, 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:

   1. I would like to be able to give the energies of my structures 
   accurate up to 0.01 kJ/mol which is about 1.10^-6 a.u. Unfortunately, I 
   only reach this level of accuracy at a cutoff of 2000 Ry… This seems quite 
   a large cutoff as I see similar calculations with cutoffs between 400-1200 
   Ry and also because the charge is already converged at 500 Ry. Am I doing 
   something wrong or is my criteria just too strict? Does this perhaps have 
   anything to do with the fact that the relative position of the atoms to the 
   grid points changes with the cutoff as mentioned by Prof. Hütter in his 
   post? 
   2. The faq <https://www.cp2k.org/faq:cutoff> and converging cutoff 
   exercise 
   <https://www.cp2k.org/events:2018_summer_school:converging_cutoff> state 
   that the PW cutoff should be large enough to properly represent the 
   Gaussian with the largest exponent, which in my system seems to be oxygen 
   with an exponent around 10.4 for the DZVP-MOLOPT-SR-GTH basis set. So is it 
   then correct to use such a complex structure (3-layer slab with adsorbate 
   and 30Å vacuum width) with "complex" options (dipole and dispersion 
   corrections) as input for my convergence tests or could I use a simpler 
   structure (e.g. an oxygen molecule in a box) with more basic settings (no 
   corrections) or would this be insufficient? 
   3. I may be planning to use larger basis sets ("regular" DZVP, TZVP or 
   TZV2P) of the MOLOPT family in the future and I have seen that the largest 
   exponents for these sets is the same as that for the smaller DZVP-SR basis 
   set (10.4 for oxygen). Is it correct then to assume that once I obtain 
   optimized cutoffs for DZVP-SR, these can be kept when switching to any of 
   the larger basis sets? 
   4. Lastly, I would like to ask for some general feedback on the input 
   for my slab calculation as this is my first calculation on CP2K and I'm 
   still getting used to all the settings. Is it okay to use 3D periodicity 
   with a large vacuum space and a dipole correction or is it better to use 2D 
   periodicity with a corresponding Poisson solver (e.g. MT, analytic or 
   wavelet)? Is it okay to use the WC functional with basis sets, 
   pseudopotentials and D3 settings optimized for PBE? 

 

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.

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