Fully periodic QM/MM simulations

[Jadzia]

Dear All, 

I'm relatively (but not completely) new to CP2K, and am currently trying to 
run a QM/MM simulation (at the moment just a test system with water) which 
is fully periodic, using DFTB3 with dispersion correction, and 
electrostatic embedding (Coulomb).

As far as I could find out, with DFTB one can set the QM box size to the 
system size, and in this case one does not need to decouple the QM images 
in the QMMM section. Does this mean that I can leave away the &PERIODIC 
section completely in the QM/MM region? Because if I have &PERIODIC 
sections in the MM and in the DFT sections, then the periodicity should 
alraedy be set up correctly, right? I have already tried to switch off only 
the &MULTIPOLE feature, but then I get an error about to a missing keyword 
related to EWALD:

Value requested, but no value set getting value from keyword GMAX of 
> section EWALD. 


Therefore I guess I need to renmove the &PERIODIC keyword in the QMMM 
section, and if I remove it the simulation seems to run fine. But I can't 
tell if the periodicity works as it should. I also read a lot of topics in 
the mailing list, but I couldn't figure things completely out. My input 
files are attached, and about any comments I would be grateful. 

Also, there is one other problem. After a while of simulation time, the QM 
water molecules are slowly falling apart. I.e. a hydrogen would detach from 
one water molecule and goes to the oxygen of another one. This happens 
almost always when MM water molecules interact with QM water molecules, but 
very few times I saw also a QM water molecule falling apart which was not 
directly near an MM water. I suspect that the* electron spillout problem* 
(as it is called in your 2005 article) is the cause. In the article is 
written that a Gaussian charge distribution is used to solve the electron 
spillout problem. Is this mechanism always active in QM/MM force 
evaluations in CP2K, or do I need to active it? (I could not find much 
about that topic.) Or do you have other recommendations on how to solve the 
water instability problem?

If you see anything other unusual in my input file I would be glad to as 
well. I have checked things as best as possible, but don't have much 
experience with CP2K. 

Best wishes,
Jadzia
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