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

[Jürg Hutter]

Hi

in any case it would be good to have a break down of energies for identical structures.

Known differences for the CP2K implementation:
1) There is no entropy term added by default. You can simulate that by using SMEARING
     and diagonalization.
2) Dipole terms in the Coulomb energy are damped in the EWALD sum in order to avoid
     problems with conditionally convergent energy contributions.

However, I assume the charges are the main problem.

regards

JH

________________________________________
From: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Magnus Rahm <magnus at compulartech.com>
Sent: Monday, September 5, 2022 11:58 AM
To: cp2k
Subject: Re: [CP2K:17606] Large discrepancy in xTB results from CP2K vs DFTB+

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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