Fully periodic QM/MM simulations

[Matt W]

Hi,

you want to keep the periodic section, if you want a periodic system. This 
_should_* make the QM region feel a potential that comes from the 
periodically replicated MM region (so essentially Ewald sum). Without that 
section you just get the potential from the point charges surrounding the 
region, that needs lots of care about boundary effects otherwise you can 
get stray fields and other artifacts.

Multipole correction is (normally a fairly small correction) that allows 
you to use a smaller QM box but still be periodic. You can ignore it if 
your QM and QMMM cells are the same size.

* I don't know if the periodic functionality works with DFTB ...

Matt

On Monday, September 4, 2017 at 3:27:49 PM UTC+1, Jadzia wrote:
>
> 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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.cp2k.org/archives/cp2k-user/attachments/20170905/e3e50f96/attachment.htm>