[CP2K-user] [CP2K:18885] Re: Problems with converging PW cutoff

[Léon Luntadila Lufungula]

Dear all,

Apparently I forgot to add my input file in my previous message, so I have 
included it as an attachment in this reply. I'm eagerly looking forward to 
your replies and thank you in advance.

Kind regards,
Léon

On Friday, 2 June 2023 at 15:44:38 UTC+2 Léon Luntadila Lufungula wrote:

> 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 
> <(469)%20391-9783>          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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