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

[Lobna Saeed]

>
> 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.


On Mon, Jun 5, 2023 at 6:59 AM Léon Luntadila Lufungula <
Leon.luntadilalufungula at uantwerpen.be> wrote:

> 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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