[CP2K-user] [CP2K:21167] Setting up GAPW for heavy elements

[Léon Luntadila Lufungula]

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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