[CP2K-user] [CP2K:17606] Large discrepancy in xTB results from CP2K vs DFTB+

[Magnus Rahm]

Hi,

> Could you send a break down of the energies for the LiF molecule for 
the two codes? 

I just realized I used to small a cell in this case. I just reran the 
calculations for the LiF molecule and now CP2K and DFTB+ results match very 
well. Apologies for the confusion. This means I have only found the problem 
in periodic systems, which might provide a clue... Please let me know if 
you would like to see any other data.

> do you have any reasons to expect that these methods must yield similar 
results? They are different and both are not so precise for the systems 
with complicated electronic effects

My understanding is that they are the same method (GFN1-xTB) with at most 
small differences in default parameters, and I expected at least 
qualitatively the same results (on par with using say, PBE, with two 
different DFT codes). Maybe I've missed something important here?

Kind regards,
Magnus


On Monday, September 5, 2022 at 11:47:06 AM UTC+2 anton.s.l... at gmail.com 
wrote:

> Dear Magnus,
>
> but do you have any reasons to expect that these methods must yield 
> similar results? They are different and both are not so precise for the 
> systems with complicated electronic effects.
>
> Yours,
>
> Anton
> 01.09.2022 12:48, Magnus Rahm пише:
>
> Dear all,
>
> I want to use CP2K (version 8.2, trying to get a more recent version 
> compiled) together with xTB for a crystal containing Li and O. I get 
> strange results already for a simple LiO2 crystal:
>
> * There is a very large discrepancy compared to DFTB+ (version 22.1).
> * Mulliken charges tend to be large, meaning that CHECK_ATOMIC_CHARGES 
> stops the SCF. If I turn it off, the system tends to converge 
> systematically to values just outside the "chemical range". Mulliken 
> charges obtained by DFTB+ are significantly smaller (and within "chemical 
> range").
> * The energy-volume curve looks strange and very different from DFTB+.
>
> I have tried converging with respect to system size and the EWALD / ALPHA 
> and GMAX parameters, but they have only a marginal impact. I have tried 
> similar calculations for a number of periodic systems. Sometimes I get 
> agreement, sometimes not. I also tried calculations for CO and NO molecules 
> which agree perfectly between CP2K and DFTB+, whereas an artificial LiF 
> molecule does not.
>
> A perhaps related issue was reported in 
> https://groups.google.com/g/cp2k/c/oFwgGcQuySs but the solutions 
> suggested there did not solve my problem.
>
> I attach input scripts for CP2K and DFTB+, as well as a figure showing the 
> E-V curve for LiO2 obtained with CP2K and DFTB+. I'm new to CP2K, DFTB+ and 
> xTB so I suspect I have made some simple mistake, and any advice is 
> appreciated.
>
> Kind regards,
> Magnus Rahm
>
>
>
>
>
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