[CP2K-user] Ionic crystals with GFN-xTB

[mpol]

Dear CP2k developers and users,

Have anyone encountered a problem with GFN-xTB when modeling ionic 
crystals? If I set

CHECK_ATOMIC_CHARGES .TRUE.

When performing CELL_OPT of a CaF_2 supercell, the program stops with the 
following warning:

*** WARNING in xtb_matrices.F:1088 :: Atomic charges outside chemical   ***
 *** range were detected. Switch-off CHECK_ATOMIC_CHARGES keyword in the ***
 *** &xTB section if you want to force to continue the calculation.      ***

Setting CHECK_ATOMIC_CHARGES .FALSE. makes the calculation divergent. As I 
get it, the charges on Ca and F become more and more unphysical in every 
SCF procedure, and, concomitantly, the total energy becomes lower and 
lower. I have attached the input and output of the divergent calculation to 
this message.

Could anyone give a hint on what to do in this situation? I am interested 
in MD of solid-liquid interfaces with ionic crystals. GFN-xTB seems the 
best choice.

With best regards,
Mikhail
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