Fully periodic QM/MM simulations

[Matt W]

Hi again,

sorry I don't really have time to give long enough replies to be clear, so 
perhaps a bit confusing.

QM-QM coupling is not needed in your case - your QM box is the same size as 
the QMMM box. It calculates multipole moments of the QM charge distribution 
and subtracts artificial interactions if the QM box is smaller than QMMM 
and adds back in correct ones.

QM-MM long range interactions as you say are the about including the ewald 
summed potential from the classical part - I think that this is not 
possible except using E_COUPL = GAUSS, which in turn requires DFT. See the 
paper from Laio et al for details.

There are several different QMMM methods available, and they have different 
ranges of applicability.

Matt

On Thursday, September 7, 2017 at 10:28:58 AM UTC+1, Jadzia wrote:
>
> Thank you for your answer Matt. 
>
> Since error messages can be very helpful if they are precise, I would like 
> summarize my understanding so far related to the error message I have 
> gotten:
>
> QM-QM long range correction not possible with COULOMB coupling.
>
>
> Firstly, from what you wrote I implicitly conclude that the periodic 
> section which I have added to my input file went into the direction of what 
> you had in mind when you wrote about including the coupling of the QM 
> region with the periodic MM images. In this case the above error message 
> which I got seems to be not fully suitable since I had disabled the QM-QM 
> long range correction (i.e. the MULTIPOLE section). To my understanding 
> (which is admittedly incomplete) the problem might be of a more general 
> nature and summarized as follows: *QM/MM long range correction not 
> possible with COULOMB coupling*
> because it seems to cover both cases QM-MM and optionally also QM-QM long 
> term corrections (if the multipole section is not switched off) for the QM 
> region. 
>
> Secondly, the error message seems to be too general in another aspect, 
> because you wrote that QM/MM periodic couplings are possible with DFT (and 
> Coulomb coupling I guess), in which case the error message does not hold in 
> its current general form. 
>
>
>
>
>
>
> On Thursday, September 7, 2017 at 9:26:55 AM UTC+2, Matt W wrote:
>>
>> As I said, I wasn't sure if there was a periodic correction implemented 
>> using Coulomb coupling for DFTB/semi-empirical.
>>
>> Looks like you just have to run without periodic coupling, or switch to 
>> DFT to use periodic coupling.
>>
>> On Thursday, September 7, 2017 at 4:41:31 AM UTC+1, Jadzia wrote:
>>>
>>> Another thing I wonder about: Matt you said that I should keep the 
>>> section QMMM periodic section so that the QM region feels the potential of 
>>> the periodically replicated MM region. Which I would call a "MM-QM long 
>>> range correction". But the error message of my last post states that 
>>> a QM-QM long range correction is not possible with Coulomb coupling when I 
>>> add this section. Therefore I wonder if the periodic section which I set up 
>>> for the MM-QM long range correction was indeed what you had in mind...
>>>
>>
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