# [QM/MM] The understanding of QM/MM method implemented in CP2K.

Xiaohui Yang shawn.yan... at gmail.com
Mon May 14 15:42:12 UTC 2018

```Dear all,

Thank you for reading this post. Here I have a few questions about the
QM/MM calculation that implemented in the CP2K.

I am planing to embed a charged QM system in the large MM box. The QM box
include one charged particles, e.g. Cl-, and some water molecules, and the
MM region is bulk water.

The first part of questions is about the fully PBC QM/MM method. Thanks to
Laino and co-workers, we can have this great method in CP2K. After reading
their articles, I am still confused about the boundary problem between QM
and MM regions.

1. The hydrogen bond in the QM region.

Because the QM box can interact with its images in the PBC, if a water
(water A) close to the boundary forms a hydrogen bond with another water
molecule in the image (water B'), the water A should feel the hydrogen bond
with water B', and has the electrostatic + vdW interaction with classical
water molecules in the vicinity. However, in a reasonable QM/MM system, the
force applied on water A isn't include the hydrogen bond with water B'.

2. The artifical "vacuum" between QM and its images.

Normally in the QM/MM system, the QM box shoud be a little bit larger than
the "optimal size" (which means the solution have correct density). If I
want to reproduce the QM region with correct density, and to prevent the QM
molecules move out of the QM box, I need to enlarge the QM size a little
and apply the "WALL" in the QM/MM section.This method is convenient and
useful. However, there is a few angstrom thickness of "wall skin", for
example, 1 angstrom (0.5 for each side). In the full PBC QM system, there
will be a kind of "vacuum" betweem QM box and its images. Therefore, the
interaction between the QM region and its images will be affected.

Because I don't fully understand above situations, I decide to employ the
isolated QM system in the PBC MM box. Here I have some questions about the
POISSON SOLVER.

1. The "POISSON" section in the DFT part.

If this section is explicitly writen in the input file, there is a warning
in the output said that this part is ignored in the QM/MM calculation, and
the electrostatic is controlled only by "PERIODIC" in QM/MM section.
However, based on my test, the poisson solver of the QM box is contorlled
by the "POISSON" section in the "DFT" section, and the "PERIODIC" in QM/MM
section does not affect the result at all.

2. The "MULTIPOLE SOLVER" and "MULTIPOLE".

As the non-PBC QM box is employed, the MT solver is prefered in my
calculation. However, the MT solver requires large QM box, which results in
high computational cost. And I don't know if the MT solver could be used
together with the "MULTIPOLE" decouple/recouple method to reproduce the
long range electrostatic potential.
I noticed that there is a "MULTIPOLE" solver, but I have no idea about it.
Can I choose this solver in the NON-PBC charged box? As far as I know, for
PBCs, the poisson solver is "periodic"; for non-PBCs, the "MT" or "WAVELET"
is required.

Thanks again for your reading, any comments and suggestions are
appreciated.

Attached is a very simple example of QM/MM input and output files to point
out the misunderstanding of "POISSON" section in both "DFT" part and "QMMM"
part. (I choose totally different periodic and solver in both sections on
purpose, and the output POISSON part shows the keyword consistent with DFT
section.)

BTW, is it possible to use OPLS/AA force field in CP2K? :D

Best Regards,

Xiaohui

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