[CP2K-user] [CP2K:16841] Re: xTB descrepancy between CP2K and DFTB+

[Jürg Hutter]

Hi

I did some calculations for your system with CP2K and I get a converged volume
of 1845. For this I did:

EPS_DEFAULT 1.0E-12
EPS_SCF 1.0E-7

&MOTION
  &CELL_OPT
    TYPE GEO_OPT
    OPTIMIZER BFGS
    MAX_DR    1.0E-04
    MAX_FORCE 1.0E-05
    RMS_DR    1.0E-04
    RMS_FORCE 1.0E-05
    MAX_ITER 100
  &END
  &GEO_OPT
    OPTIMIZER BFGS
    MAX_DR    1.0E-04
    MAX_FORCE 1.0E-05
    RMS_DR    1.0E-04
    RMS_FORCE 1.0E-05
    MAX_ITER 50
  &END
&END

I did some additional checks on parameters, but there was no change compared
to the defaults.

This points to a difference of ca. 55 A3 wrt to the DFTB+ results (1900). This is a difference
of 2.5% in volume or ca. 1 % in cell length.

As said in the last post, if you want to investigate further you have to look into the
results of both codes at identical structures.
Compare energies (total, and partial), forces and stress.

regards

Juerg Hutter

________________________________________
From: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Xavier Bidault <jazzquark at gmail.com>
Sent: Monday, April 11, 2022 4:34 AM
To: cp2k
Subject: [CP2K:16831] Re: xTB descrepancy between CP2K and DFTB+

I used the driver/optimizer couple GO/rational for DFTB+ and explored a few SCC epsilon parameters (expressed below as 1e-n). For CP2K with BFGS optimizer, I similarly explored various values for SCF epsilon. Epsilons for geometry of 1e-5 or 1e-6 yields very similar results.

CP2K (all other epsilons are default):
n   Volume(A3)
5   1870.51
6   1863.46
7   1873.30
8   1759.92
9   1834.73
10 1944.21
11 1761.18
12 1846.56
13 1860.54
14 1826.25
15 1852.36

The differences are large, and non-monotonic. There is, imo, no obvious choice. I also quickly checked that without constraining the monoclinic geometry, it was actually preserved.

DFTB+:
5   1901.99
6   1859.22
7   1904.44
8   1902.72
9   1900.05
10 1900.89
11 1901.01
12 1900.90
13 1900.35

Note that the only one preserving the monoclinic geometry is 1e-6. For all the other epsilons, Alpha and Gamma are around 90.2° and 90.7°, respectively, instead of 90°. But all those other volumes are very similar. Would it mean we have to avoid 1e-6 for DFTB+? Or the contrary?

Any remarks that could help?

On Thursday, April 7, 2022 at 5:05:34 PM UTC-5 Xavier Bidault wrote:
Hello,

I just posted also on DFTB+ issues (https://github.com/dftbplus/dftbplus/issues/1001), including the input script for DFTB+

Xavier

On Tuesday, April 5, 2022 at 3:22:02 PM UTC-5 Xavier Bidault wrote:
Hello,

I run a variable-cell optimization of a high-energy molecular crystal, beta-HMX, with CP2K 9.1 and DFTB+ 21.2, both using GFN1-xTB, which includes D3(BJ) correction. To compare apples to apples, I optimized the same supercell, using the Gamma point. The initial (experimental) volume for the supercell is 2069.80 A3. The resulting volume is:
1950.83 A3 with DFTB+
1759.92 A3 with CP2K

In order to check if I did something wrong in the configuration files, I used DFTB + D3(0) with the default DFTB parameter file from CP2K for both CP2K and DFTB+, and I find:
1467.79 A3 with DFTB+
1467.14 A3 with CP2K
So, DFTB + D3(0) is consistent between CP2K and DFTB+, but not GFN1-xTB.

IMO, the dispersion correction of xTB is wrong somewhere in CP2K. Looking at the difference between CP2K and DFTB+, could it be that D3(BJ) correction is counted twice in CP2K? Or with some wrong units we don't have access to?

I attached the CP2K input file for the GFN1-xTB calculation.

Please, let me know.

Thank you,
Xavier



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