[CP2K-user] [CP2K:15690] Re: SCF convergence problem with large basis sets

Tom Braeckevelt tom.bra... at ugent.be
Tue Jul 6 11:32:47 UTC 2021

Dear Nick,

Thank you very much for the helpful information. Currently, I am using as 
reference value the energy difference between the two structures obtained 
with VASP and a large plane-waves energy cutoff. What I noticed, is that in 
both the MOLOPT and regular basis sets the value approaches the VASP one 
with increasing basis set size, with the MOLOPT ones performing similarly 
to the regular ones except for the TZVP, in which the regular one performs 
better than the MOLOPT.

[image: BasisSet_Comparison.png]

Thus, I wanted to check which value I could get from a QZV3P basis set in 
order to have a complete picture of the basis set size effect on the energy 
differences (of course I would later on resort to smaller basis set in an 
MD simulation for time reasons).   

I will try to increase the cutoff and see if I can get a converged 
wavefunction, I was not aware of this effect related to the CUTOFF.


Thanks again, best regards,


On Tuesday, July 6, 2021 at 12:33:21 PM UTC+2 ma... at gmail.com wrote:

> Thank you Prof. Hutter. This is very helpful!
> Regards,
> Hongyang
> 在2021年7月6日星期二 UTC+10 下午6:04:35<jgh> 写道:
>> Hi 
>> in GAPW calculations the electron density on the regular grids depends 
>> on the soft/hard basis separation. This value will converge to some 
>> irregular value, but it is impossible to predict it. 
>> More important is the total electron density. If it is off from the 
>> integer number of electrons, you see errors from two sources. 
>> 1) the cutoff from the regular grids (as in GPW) 
>> 2) the atomic size approximation 
>> Errors from 1) can be reduced by increasing the cutoff, errors from 
>> 2) are related with the rcut value of the ALL_ELECTRON file. 
>> "Carefully" adjusting that value can improve high precision results. 
>> regards 
>> Juerg Hutter 
>> -------------------------------------------------------------- 
>> Juerg Hutter Phone : ++41 44 635 4491 <+41%2044%20635%2044%2091> 
>> Institut für Chemie C FAX : ++41 44 635 6838 <+41%2044%20635%2068%2038> 
>> Universität Zürich E-mail: h... at chem.uzh.ch 
>> Winterthurerstrasse 190 
>> CH-8057 Zürich, Switzerland 
>> --------------------------------------------------------------- 
>> -----c... at googlegroups.com wrote: ----- 
>> To: "cp2k" <c... at googlegroups.com> 
>> From: "ma... at gmail.com" 
>> Sent by: c... at googlegroups.com 
>> Date: 07/06/2021 01:56AM 
>> Subject: [CP2K:15690] Re: SCF convergence problem with large basis sets 
>> 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 
>> <(517)%20337-0396> same as CUTOFF of 50 Ry. I'm wondering do I need to 
>> worry about this when I use ALL_ELECTRON basis sets? 
>> Regards, 
>> Hongyang 
>> 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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