[gmx-developers] Question about MM point charges in QM/MM calculation

[Berk Hess]

But I was trying to say that with PBC it does not make sense to use all 
MM charges.
Since your system is periodic all charges are periodically repeated in 
all directions.
To do this correctly you would need infinitely many charges.
You could use the minimum image convention, but then you will have a 
rectangular brick
of MM charges. When a charge on the boundary moves slightly, it will 
suddenly jump
to the other side.

I would suggest to simply use a cut-off of nearly half the smallest box 
size.
Then you will have a nearly spherical group of charges.

Berk

Pia Toelle wrote:
> Thanks for your help!
>
> I do have pbc! And I would like to include all non-QM-atoms as MM point
> charges. As I am using DFTB, a big number of point charges are no problem.
> What's about the last comment? Is it possible to put all cut-off's to
> zero and where? Are you talking about changes in the code or just in the
> mdp-file?
> Or maybe I will try the changes Gerrit proposed.
>
> Pia
>
> Berk Hess wrote:
>   
>> But in case of no pbc, couldn't you just run with no cut-off (all
>> cut-offs set to zero)
>> and automatically get all MM charges?
>> Since MM should be cheap compared to QM this should not be
>> computationally expensive.
>>
>> Berk
>>
>>