[CP2K-user] [CP2K:17602] Re: Large discrepancy in xTB results from CP2K vs DFTB+

[Jürg Hutter]

Hi

thank you for testing. Could you send a break down of the energies for the LiF molecule for
the two codes? That might help to recognize the source of the difference.

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 8:40 AM
To: cp2k
Subject: [CP2K:17599] Re: Large discrepancy in xTB results from CP2K vs DFTB+

For the record, the problem is the same in CP2K version 2022.1.

On Thursday, September 1, 2022 at 12:48:35 PM UTC+2 Magnus Rahm wrote:
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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