DFTB3 within QM/MM simulations

Jadzia c.gor... at gmail.com
Fri Sep 15 10:05:26 UTC 2017

For a further test, I made a smaller water box with only 22 Angstrom box 
edge length this (before 42 Angstrom). 

I adapted the QM Poisson parameters to:

       EWALD_TYPE spme
       ALPHA 0.3
       GMAX 11 11 11
       O_SPLINE 5
      &END EWALD

Now the waters are falling apart again. GMAX I have set to half of the box 
edge lengths, because this worked in the previous test system.

My guess is that the QM/MM electrostatic interaction somehow disturbs the 
QM density in an unnatural way. The question is can this be fixed?

Best wishes,

On Friday, September 15, 2017 at 6:35:25 AM UTC+2, Jadzia wrote:
> 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
>>        GMAX 25
>>        O_SPLINE 5
>>       &END EWALD
>> 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
>>        GMAX 42 42 43
>>        O_SPLINE 6
>>       &END EWALD
>> 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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