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

[Wei Lai]

With my limited knowledge of fortran, I went through source codes of DFTB+
and cp2k and might have an idea for the difference in results.

First, the construction of H0, S, and gamma matrix seem to be consistent
between two codes.  However, it seems they differ on how SCC
(self-consistent charge) was applied.  In DFTB+, the self-consistent charge
was the Mulliken charge on atoms.  In cp2k, it seems that H0, S, and gamma
matrix was fed into KS-DFT framework so the self-consistent charge was the
electron density in the real space.  Ideally this needs the basis functions
which are not contained in skf files so perhaps some default Gaussian basis
functions were used.  If the previous observation is correct, there seems
to be an issue in the implementation of SCC-DFTB in cp2k.

Thanks, Wei

On Mon, Dec 24, 2018 at 4:09 PM Wei Lai <wei.w... at gmail.com> wrote:

> With the intention to help with the discussion, I took the liberty to post
> the following numbers.
>
> If I turn off “minimum image convention” or “mic” in qs_neighbor_lists.F
> (version 5.1; this was done automatically in version 6.1), the periodic
> nonscc results of cp2k are close to those from dftb+.
>
> nonscc total energy:  -14.90396117681518 (cp2k) vs -14.9042897104 (dftb+).
>
> Maxime was using 6.0 development version and the nonscc total energy
> was -15.00558399676375.
>
> However, the difference in the periodic scc case became more dramatic:
>
> core energy: -14.7017837002 (dftb+), -11.35591447406040 (cp2k, alpha=0.3),
> -13.03486875813706 (cp2k, alpha=0.5), -13.89182287767537 (Maxime, alpha=0.3)
> scc energy:  1.2109415266 (dftb+), 2.94796400907580 (cp2k, alpha=0.3),
> 2.06305475569169 (cp2k, alpha=0.5), 0.07851968376930 (Maxime, alpha=0.3)
>
> Thanks, Wei
>
>
> On Mon, Dec 24, 2018 at 6:08 AM Thomas Kühne <tku... at gmail.com> wrote:
>
>> Dear Maxime,
>>
>> your input files looks good to me.
>> How large is the difference in the
>> periodic cases?
>>
>> Merry Christmas,
>> Thomas
>>
>> Am 22.12.2018 um 22:45 schrieb Maxime Van den Bossche <
>> maxime.cp.v... at gmail.com>:
>>
>> 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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