[CP2K-user] [CP2K:11082] comparing CP2K-DFTB (5.1) and DFTB+ (18.2)

[hut... at chem.uzh.ch]

Hi

I looked into these problems and I see that this behavior is
from a combination of bugs and hidden constraints on variables.

The bugs were introduced when the MIC was dropped after the implementation
of the k-point code. The Ewald parameters are partly set from the
input and partly set by using default values within the code.

To be on the save side I would suggest to only do DFTB calculations
with large enough computational cells. You can use
the MULTIPLE_UNIT_CELL keyword if you have a small cell.
   &TOPOLOGY
      MULTIPLE_UNIT_CELL ${NREP} ${NREP} ${NREP}
    &END
    &CELL
      ABC 4.0 4.0 4.0
      MULTIPLE_UNIT_CELL ${NREP} ${NREP} ${NREP}
    &END CELL

I guess a cell with 10 Angstrom box length should be ok.

Use an ALPHA value that is 1 or larger and adjust GMAX for
your box size to get converged values.

I am working on fixes, they should be ready in the TRUNK version soon
and will be in the next release.

best regards

Juerg

--------------------------------------------------------------
Juerg Hutter                         Phone : ++41 44 635 4491
Institut für Chemie C                FAX   : ++41 44 635 6838
Universität Zürich                   E-mail: hut... at chem.uzh.ch
Winterthurerstrasse 190
CH-8057 Zürich, Switzerland
---------------------------------------------------------------

-----cp... at googlegroups.com wrote: -----
To: "cp2k" <cp... at googlegroups.com>
From: "Maxime Van den Bossche" 
Sent by: cp... at googlegroups.com
Date: 12/22/2018 10:45PM
Subject: Re: [CP2K:11082] comparing CP2K-DFTB (5.1) and DFTB+ (18.2)

Hi all,

I also did a quick comparison now between CP2K and DFTB+
for a simple rocksalt MgO structure (8 atoms in total,
using the OT minimizer in CP2K). The input and output 
files for both codes are attached. 

What I found so far:

* Without periodicity, the two codes agree quite well
  in terms of the H0 and SCC energies (up to ~1e-4 Ha)
  as well as the Mulliken charges.

* In the periodic case, so far I haven't managed to
  get comparable results with the two codes. The CP2K
  results indeed appear sensitive to the choice of the 
  Ewald alpha parameter, similar to what Wei observed.
  For each of the three ALPHA values I tried, the resulting
  energies and SCC charges are quite different from DFTB+.

* In contrast to Wei, though, I also get different results
  for both codes in non-self-consistent calculations
  (but at least here the CP2K results indeed no longer
  depend on ALPHA).

It would be great if you could check whether my CP2K
inputs have been appropriate. If so, I hope this example will
help to get to the bottom of this.

Best,
Maxime  
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[attachment "files.zip" removed by Jürg Hutter/at/UZH]