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

ma...@gmail.com ma455... at gmail.com
Mon Jul 5 23:56:29 UTC 2021

Hi Nick,

Just curious about your third suggestion (the CUTOFF issue). 
When I use ALL_ELECTRON basis sets, I found increasing CUTOFF does not 
change the second value of "Electronic density on regular grids" at all, 
although the SCF converged energy is decreasing. I tried DZVP all electron 
basis set for Si. But even using a CUTOFF as high as 1000 Ry, the second 
value of "Electronic density on regular grids" is still 13.5173370396 same 
as CUTOFF of 50 Ry. I'm wondering do I need to worry about this when I use 
ALL_ELECTRON basis sets?


在2021年7月6日星期二 UTC+10 上午8:49:27<n... at berkeley.edu> 写道:

> 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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