[CP2K-user] [CP2K:18899] Re: Problems with converging PW cutoff

[Léon Luntadila Lufungula]

Dear all,

I have just read in another post 
<https://groups.google.com/g/cp2k/c/OGyF0D6tUQQ/m/qPsP6VulAgAJ> that I 
should include the PBE correlation functional in the XC section when using 
the WC functional 
<https://journals.aps.org/prb/abstract/10.1103/PhysRevB.73.235116>, i.e.,

&XC_FUNCTIONAL
    &GGA_X_WC
    &END
    &GGA_C_PBE
    &END      
&END XC_FUNCTIONAL

Can someone confirm that this is how you should use the WC functional and 
could this influence the results of my convergence tests?

Kind regards,
Léon
On Monday, 5 June 2023 at 12:15:40 UTC+2 Léon Luntadila Lufungula wrote:

> Dear Prof. Hütter,
>
> Thanks for the clarification on Lobna's point about the total energies as 
> compared to those calculated in other codes. Do you perhaps have any tips 
> or remarks regarding my initial questions? I am quite stuck in my progress 
> until I find a way to properly converge my cutoffs and I think you're 
> expertise might help me get past this problem. 
>
> Thanks in advance!
>
> Kind regards,
> Léon
>
> On Monday, 5 June 2023 at 11:50:56 UTC+2 Jürg Hutter wrote:
>
>> Hi 
>> please only compare codes for quantities that can be transferred. For 
>> example, it is almost 
>> impossible to compare total energies. Relative energies or other 
>> properties can be 
>> compared for equivalent settings. 
>> If you have identified a pseudopotential/basis set problem for CP2K, 
>> please provide us with the 
>> necessary information and we will investigate. 
>> Thank you 
>> JH 
>>
>> ________________________________________ 
>> From: cp... at googlegroups.com <cp... at googlegroups.com> on behalf of Lobna 
>> Saeed <lobna... at gmail.com> 
>> Sent: Monday, June 5, 2023 9:18 AM 
>> To: cp... at googlegroups.com 
>> Subject: Re: [CP2K:18885] Re: Problems with converging PW cutoff 
>>
>> 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<mailto: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><tel:(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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