DFTB3 within QM/MM simulations

[Jadzia]

I have contacted one of the developers of DFTB3. He has also much 
experience with DFTB3 QM/MM water simulations, and he said that even though 
DFTB3 has a few problems with water, these should not cause them to fall 
apart. 

Therefore there seem to be two possibilities:

   1. Something is wrong with the setup. 
   2. There is a bug in CP2K. 

I have tried my best to check if something is wrong with the setup, but I 
couldn't find anything. Also, I have run a lot of simulations with 
different settings. 
If it is a bug in CP2K I cannot tell, this might be a question for the 
developers. 

Best wishes,
Jadzia


On Wednesday, September 13, 2017 at 10:38:16 AM UTC+2, Jadzia wrote:
>
> Hi everyone,
>
> I'm trying to use DFTB3 as the QM method of a QM/MM simulation of water 
> (which is only a test system).
>
> However, my QM water molecules are falling apart. At first some H20 
> molecules form H3O+ and OH-, and then it goes on to form H4O++ and O--. All 
> this happens between QM atoms only. 
>
> I have adjusted and switched many QM and QMMM parameters (see also my last 
> topic here <https://groups.google.com/forum/#!topic/cp2k/EmRoMadjbRw> which 
> was intended to get the same simulations running with fully periodic QM/MM 
> PBC). As I have found out, it seems that the Hubbard derivative (which is 
> to my knowledge the DFTB3_PARAM in CP2K) is related to the problem. If I do 
> not specify this parameter for the H and O atoms (it is set to 0 by default 
> by CP2K) then my waters are not falling apart. The Hubbard derivatives I 
> have used are the ones provided by dftb.org. 
>
> Do you think it is fine to run the simulations without specifying the 
> Hubbard derivatives? (i.e. using the default value of 0) 
> And do you have any other suggestions or comments on why this problem 
> could arise or solved?
>
> If I run QM-only simulations with the same DFTB3 settings and with the 
> official Hubbard derivative parameters from dftb.org, then the 
> simulations run just fine. Only when enabling QM/MM my QM water falls apart.
>
> However, I now found out that even without Hubbard derivatives I manage to 
> let the water fall apart, namely when I change the Poisson settings of the 
> QM region. With the following QM Poisson settings the water does not fall 
> apart:
>
>     &POISSON
>       &EWALD
>        EWALD_TYPE SPME
>        GMAX 25
>        O_SPLINE 5
>       &END EWALD
>     &END POISSON
>
> The entire input file is attached as well. When changing the Poisson 
> settings to values which seem to be more optimal:
>
>     &POISSON
>       &EWALD
>        ALPHA .44
>        EWALD_TYPE SPME
>        GMAX 42 42 43
>        O_SPLINE 6
>       &END EWALD
>     &END POISSON
>
> then the QM water falls apart. Which I find strange since these Poisson 
> settings are more "sound" because they are more close to what is 
> recommended. E.g. the box size is 42 42 53 Angstrom, and it is recommended 
> to use 1 per Angstrom for GMAX. Also, O_SPLINE was set before to 5, which 
> can lead to an interpolation problem to my knowledge. 
>
> Looking forward to hearing from you and your recommendations. 
>
> Best wishes,
> Jadzia
>
>
>
>
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