[CP2K-user] [CP2K:21369] Re: Setting up GAPW for heavy elements

[Léon Luntadila Lufungula]

Dear all,

I really request some help with this as I have worked on this problem on 
and off for the last couple of months and still haven't been able to make 
any progress... I need all-electron calculations to proceed with my own 
research as well as for the research of my two students who are eagerly 
waiting for me to solve this, so I would greatly appreciate any help at all.

I started out with testing GAPW with the default settings for some small 
organic molecules (H2, CH4, NH3, H2O, Cl2, SiH4, PH3 and H2S) containing 
the elements of the adsorbates I intend to use during my research along 
with TiCl4 for titanium and comparing the GAPW energies, forces, bond 
lengths and angles to their counterparts calculated with Gaussian 09. To be 
able to make this comparison, I have used the def2-TZVPP basis set as this 
is available in both CP2K and G09. The only changes to the defaults I had 
made was the values of the HARD_EXP_RADIUS, which I took from the table in 
the supplementary information of the following paper co-authored by Jürg 
Hutter <https://pubs.acs.org/doi/10.1021/acs.jctc.4c00614?ref=pdf>. 
Nevertheless, the results were promising as the differences with the G09 
results were small but the errors seemed to grow as I went down the 
periodic table (see below or attached image 'Small molecules GAPW.png) and 
the same trend was seen for  the electron counts ("Total charge density 
(r-space)" and "Total Rho_soft + Rho0_soft (g-space)"). I have attached the 
TiCl4 input and output files as an example.
[image: Small molecules GAPW.png]
Unfortunately, testing with small molecules containing Zr and Hf was a bit 
more complicated because the def2-tzvpp basis sets use ECP for Zr and Hf, 
which is not compatible with CP2K.

Now I wanted to try and use GAPW for my titania surface slab, but I have 
tried a lot of different combinations for the GAPW settings but none have 
allowed me to converge my 3x1x3 anatase (101) slab. The only calculation 
that has converged was of a 1x1x1 slab (see attached files 
a101-1x1x1-gapw-molopt-tzvpp-all1.inp|.out) where I used the settings Jürg 
Hutter proposed in an earlier post of mine 
<https://groups.google.com/g/cp2k/c/6qjk0K5oRqU/m/YL83SHAIBgAJ>, but even 
here the electron counts are around 3E-6 which seems quite large to me. 
Using the same parameters but using a 3x1x3 slab, the calculation starts 
with electron counts starting around 3E-5 for the first 2 inner SCF loops 
and seems to go towards convergence at the end of the 2nd loop, but then 
goes haywire in the 3rd SCF loop and never reaching convergence (see 
attached files a101-3x1x3-gapw-molopt-tzvpp-all1.inp|.out). I have tried a 
lot of different values for HARD_EXP_RADIUS for both O and Ti and found out 
that 1.30 and 1.80, respectively, resulted in the lowest electron counts 
for the 1x1x1 slab, but again when running the 3x1x3 slab the first 2 inner 
SCF loops look promising, but from the 3rd one onward it goes nowhere (see 
attached  files a101-3x1x3-gapw-molopt-tzvpp-all2.inp|.out).

I'm really at my wit's end here, I have tried a lot of different settings 
and none seem to work and I have no clue why it isn't working or how I 
should tackle this problem in a sytematic way. I also can't compare to G09 
energies so I only have the electron counts to go on, but there isn't a 
clear trend about how to improve these with the GAPW settings available. I 
would love it if someone could point me in the right direction so I can 
finally get past this hurdle. The questions in my previous post in this 
topic still stand and would love to get some insight into them if possible.

Greetings,
Léon

On Tuesday, 18 February 2025 at 10:31:22 UTC+1 Léon Luntadila Lufungula 
wrote:

> Dear all,
>
> I have been trying my hand at performing GAPW calculations for the last 
> couple of weeks and I have made due with the defaults for the first-, 
> second- and thir-row elements I plan on using during my calculations. 
> However, elements from the fourth row or higher (Ti, Zr and Hf in my case) 
> are a bit harder to get right.
>
> I've scoured the mailing list and have done a lot of test calculations, 
> but I'm still unsure how to optimize all of the parameters in a systematic 
> way. Let me quickly sum up what I know about the different parameters and 
> then ask what is still unclear.
>
> *QS/EPSFIT and KIND/HARD_EXP_RADIUS:*
> Both of these parameters shift the border between the soft and hard part 
> of the basis set and have an inverse relationship with the "cutoff" 
> exponent. Larger values of either parameter lowers the cutoff exponent and 
> will thus result in a soft basis set that is softer (i.e., lower 
> exponents). I'm quessing HARD_EXP_RADIUS cannot be too large so as to keep 
> the hard density confined. Matthias Krack suggested to lower EPSFIT to 
> 1.0E-6 in another post 
> <https://groups.google.com/g/cp2k/c/FUvjXq9RvWY/m/2xSxfzuVAgAJ> and Jürg 
> Hutter stated here 
> <https://groups.google.com/g/cp2k/c/JX8bDi_w478/m/tjLIF_UUAQAJ> that the 
> max exponent of the soft basis should generally be in the range of 5-8. My 
> questions here are:
>
>    1. When using the defaults, I generally get max exponents between 1-5 
>    for the soft basis. Should I make sure that the max exponents are in the 
>    range 5-8?
>    2. Should you start with lowering EPSFIT (global parameter) with the 
>    default HARD_EXP_RADIUS values and then optimize the HARD_EXP_RADIUS 
>    (kind-specific parameter) for each element in the system?
>    3. Since EPSFIT and HARD_EXP_RADIUS are both connected to the basis 
>    set, should you do this optimization for each basis set individually or can 
>    you do it for one and then apply it to other basis sets?
>    
> *EPSISO and EPSRHO0:*
> Jürg Hutter stated here 
> <https://groups.google.com/g/cp2k/c/n70_s_Tbt1A/m/LFlsSmA4BgAJ> that 
> EPSISO is useful for heavy elements, but did not state whether to lower or 
> increase it to get better results. Matthias Krack also wrote here 
> <https://groups.google.com/g/cp2k/c/FUvjXq9RvWY/m/8VwHvHGUAgAJ> that 
> reducing EPSRHO0 will lead to better accuracy. My questions here are:
>
>    1. I only found these two suggestions of altering these parameters. Is 
>    it worth playing around with them or should I keep them at the default?
>    2. If I want to alter them, do I do this after EPSFIT and 
>    HARD_EXP_RADIUS and do I just use the electron counts (i.e.,  "Total charge 
>    density (r-space)" and "Total Rho_soft + Rho0_soft (g-space)") to check 
>    which values give the best results?
>
> *LEBEDEV_GRID and RADIAL_GRID*
> It has been said many times over in the mailing list to increase these 
> values for heavier elements as Matthias Krack 
> <https://groups.google.com/g/cp2k/c/FUvjXq9RvWY/m/8VwHvHGUAgAJ> said as 
> well in an earlier post. My question here are:
>
>    1. Marcella Iannuzi stated in 2014 
>    <https://groups.google.com/g/cp2k/c/IH5k1SdMu5k/m/kZz4QxbupK8J> that 
>    LEBEDEV_GRID is usually in the range 50-80 and RADIAL_GRID in the range 
>    100-200. Is this still accurate? I've seen Jürg Hutter use a 100/100 
>    combination for titanium in a previous post of mine 
>    <https://groups.google.com/g/cp2k/c/6qjk0K5oRqU/m/YL83SHAIBgAJ>.
>    2. How do I know if these values are too small? Do I start with them 
>    at 80/200 and then optimize EPSFIT/HARD_EXP_RADIUS and then reduce the 
>    LEBEDEV_GRID/RADIAL_GRID values until I see an increase in the electron 
>    counts?
>
> *LMAXN0, LMAXN1 and ALPHA0_HARD:*
> I feel like I shouldn't mess around with these parameters but I have seen 
> Jürg Hutter define a larger LMAXN0 for Ti in an earlier post of mine 
> <https://groups.google.com/g/cp2k/c/6qjk0K5oRqU/m/YL83SHAIBgAJ> and seen 
> ALPHA0_HARD set to 10 in several occasions, for example in the GAPW class 
> <https://www.youtube.com/watch?v=L0hKLjvjIFU&t=1754s> on youtube from 
> 2019.
>
>    1. Should I leave these at the defaults or try and increase LMAXN0 and 
>    set ALPHA0_HARD to 10?
>
> My apologies for the long post with a lot of questions, but I feel like 
> some of these details are still missing from the documentation and mailing 
> list and I know that a proper answer would be immensely helpful for people 
> like me who want to use GAPW for non-standard (heavy) elements and run into 
> trouble with the defaults.
>
> Kind regards,
> Léon 
>

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