[CP2K:3793] Poisson solvers in QMMM

[Frank Uhlig]

Dear Teodoro,

thanks for the answer. It is not true that "nothing prevents [me] from
implementing" the recoupling for different Poisson solvers. As you may
understand, there are certain constraints given by work and personal
obligations. If _you_ decide to do it all in your free time that is your
choice and also highly appreciated. But please, remember that it takes time
to get familiar with the code, cp2k is a rather large project and not all
of us are familiar with FORTRAN, good programmers and might have to use
cp2k as a black box. This brings me to issue on hand, as a
"cp2k-black-box-user" I noticed several inconsitencies in what is written
in the reference manual and what is happening during the calculation:

The section %FORCE_EVAL%QMMM%PERIODIC is classified as "cannot be optional"
in the manual. If you take the test from the before mentioned post, you can
easily check that this section behaves differently and also results will
differ depending on its presence in the input file.

The section %FORCE_EVAL%QMMM%PERIODIC %MULTIPOLE is classified as "can be
optional" and the explanation of its purpose is:

"This section is used to set up the decoupling of QM periodic images with
the use of density derived atomic point charges."

Whether this section is included or not does not change the resulting total
energy and in both cases the output states "Recoupling energy [...]".

Those two things, seem to be either bugs or errors in the input
description. I would appreciate if you could comment on what is what.

Furthermore, the Wavelet solver can in principle be employed without
further implementations. When one sets the corresponding periodicity in the
DFT settings to XYZ the calculation will be equivalent to a plain, periodic
plane-wave calculation and the de- and re-coupling can be used for that and
is fully equivalent to using the Multipole solver (also really gives fully
equivalent results).
CP2k in some places gives error messages for "illegal combination[s]" of
keywords and in my opinion it would be helpful to get such an error message
for the combination of the Wavelet solver (for isolated system) and what
seems to enable recoupling in the input. Btw, as far as I know, your 2006
JCTC publications about the QMMM boundary conditions does not mention that
the recoupling is not possible for other Poisson solvers and the behaviour
of CP2k with respect to the input is just a bit deceiving.

Thanks in advance and best regards,

Frank

On Apr 20, 2012 5:23 PM, "Teodoro Laino" <teodor... at gmail.com> wrote:
>
> Dear Frank,
>
> you have already found the reasons. In order to have recoupling the only
poisson solver working at the moment is multipole.
> Nothing prevents you from implementing also for the other solvers
something similar.
> If instead you want to use cp2k as a black box, you have to stay within
the implementation borders (see papers and regtests input provided):
everything beyond that is not guaranteed to work.
> Regards,
> Teo
>
> On Apr 20, 2012, at 4:56 PM, Frank Uhlig wrote:
>
>> Dear CP2k'lers,
>>
>> related to a previous post (
https://groups.google.com/d/topic/cp2k/iMsqLpZnRyQ/discussion) I just
wanted to highlight the main problems again, because they might have been
hidden in the mess.
>>
>> The QMMM electrostatic coupling is different for different Poisson
solvers if analogous inputs with Wavelet and Multipole solver are used. In
the example from the previous post (using Multipole solver) the QM images
are recoupled. If one just changes the Poisson solver to the Wavelet
solver, no recoupling of the QM images is performed. This behaviour then
further depends on the inclusion/exclusion of the non-optional section
%FORCE_EVAL%QMMM%PERIODIC.
>> The input can be changed in a way that one reproduces the total energies
with both Poisson solvers (up to ~0.01 eV).
>>
>> This behaviour is counterintuitive, imo, and I would appreciate comments
of the developers, if it is intended, if I am doing something wrong etc pp
>>
>> Cheers,
>>
>> Frank
>>
>>
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