[CP2K-user] SCF convergence problem with large basis sets

[Nicholas Winner]

Tom,

(1) When you say that you had no issues with  DZVP -> TZVP -> TZV2P basis 
sets, were you using the MOLOPT basis sets? The MOLOPT basis sets were 
optimized using the overlap matrix condition number as a constraint in 
order to make them more numerically stable. This is why they are the basis 
set type of choice for condensed phases. If you *were* using MOLOPT, then 
once you move to the QZVP basis sets, for which there are no molopt basis 
sets, then that is why it become harder to converge. If you *were not* 
using the MOLOPT basis sets I would encourage you to stick with them. 
Generally, condensed matter systems have converged properties around TZVP 
quality in my opinion, and DZVP is still pretty good. If you need true 
chemical accuracy, then you're going to need to move beyond DFT anyway.

(2) There is a general issue of using larger basis sets, which is the 
nature of the Gaussian type orbitals. GTOs are not an orthonormal basis, 
unfortunately, so the larger your basis set, the greater the risk of 
introducing linear dependencies that make converge very difficult. Another 
reason to limit the size to only as large as you need for your application. 
Beyond linear dependencies, the conditioner number itself also increases 
with increasing basis set size.

(3) Your CUTOFF in your multi-grid is not converged. I noticed this because 
after your SCF loops you have the line "Electronic density on regular 
grids. -605.9997903253        0.0002096747" -- the second number in this 
column should be <1e-8 preferably. The cutoff is the most common cause for 
this, and your cutoff of 350 is not sufficient. To determine the cutoff, 
take the largest exponent in your basis set and multiply it by the relative 
cutoff. Your CUTOFF value should be at least this large, otherwise your 
multigrid will not be able to accommodate the hardest exponents. Oxygen has 
an exponent of ~12 at the QZV3P level of theory, so your CUTOFF should be 
around 480 if you are using the default REL_CUTOFF of 40. The exponents 
change with your basis set, so this could be part of the issue you were 
facing when you got to the larger ones.

Try fixing your CUTOFF value and see if it helps, but also consider using 
smaller basis sets, maybe of the MOLOPT type, which are generally 
sufficiently accurate for most DFT calculations. 


-Nick
On Monday, July 5, 2021 at 5:02:47 AM UTC-7 tom... at ugent.be wrote:

> Sorry forgot the attachments.
>
> On Monday, July 5, 2021 at 1:44:50 PM UTC+2 Tom Braeckevelt wrote:
>
>> Dear CP2K users/developers,
>>
>> I was performing some benchmarks related to the basis set size (with 
>> simply PBE-D3). In particular, I was checking the energy difference between 
>> benzene adsorbed in the H-SSZ-13 zeolite and its protonated counterpart. 
>> While I can see a steady improvement going from DZVP -> TZVP -> TZV2P 
>> (without any issue in the calculation) I encountered large problems in the 
>> SCF convergence when using even larger basis sets, like the QZV2(or 3)P and 
>> the augmented basis sets. At this point, I was only able to converge a 
>> couple calculations with the QZV3P using the CG optimizer (normally I use 
>> the DIIS) and the FULL_KINETIC preconditioner (I normally use the 
>> FULL_SINGLE_INVERSE), which according to the manual should be more robust. 
>> I also tried the diagonalization method instead of the OT one but also in 
>> that case the scf was not converging.
>> The problem is that even with the two converged calculations the energy 
>> difference between the two species I'm studying results to be ~3000kJ/mol 
>> vs the expected ~100kJ/mol, thus it looks like the two scf have converged 
>> to different minima.
>>
>> Does anybody know how to robustly converge the scf with large basis sets? 
>> In attachment the input and output of the supposedly converged calculations.
>>
>> Thank you in advance,
>> Tom
>>
>
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