[CP2K-user] [CP2K:18887] Re: Problems with converging PW cutoff
Léon Luntadila Lufungula
Leon.luntadilalufungula at uantwerpen.be
Mon Jun 5 09:27:03 UTC 2023
Dear Lobna,
Thanks for the reply!
I can see that the total energies may differ from those obtained with other
QC software packages as the computational methods (implementation, basis
set, etc.) might be different, but I would think that this does not
influence the convergence of the total energy as a function of the cutoff.
I could indeed compare the lattice parameters of a calculated anatase unit
cell at different cutoffs with those found experimentally, but wouldn't it
be computationally expensive to check as I would have to do CELL_OPT
calculations for each cutoff as opposed to simple ENERGY calculations?
Also, should thermal effects be taken into account in some way as CELL_OPT
calculations are at 0K and the experimental CIF was recorded at RT?
You also mention that Ti requires an optimization of the basis set and
pseudopotentials, but I don't know how I would go about doing this
optimization, do you have any suggestions?
Kind regards,
Léon
On Monday, 5 June 2023 at 09:19:01 UTC+2 Lobna Saeed wrote:
> 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.luntad... 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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