[CP2K-user] [CP2K:17585] Large discrepancy in xTB results from CP2K vs DFTB+

[Magnus Rahm]

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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