[CP2K-user] [CP2K:18895] Re: Problems with converging PW cutoff

[Léon Luntadila Lufungula]

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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