[CP2K-user] [CP2K:17607] Re: Large discrepancy in xTB results from CP2K vs DFTB+

[Xavier Bidault]

I recently run variable-cell optimization of various molecular crystals and
I found xTB at CP2K ultra sensitive to EPS_DEFAULT. Tested from 1e-5 to 1e-24
(with EPS_SCF 1e-8), and no convergence happened. I just ended up with
EPS_DEFAULT 1e-10 as a "gut" choice. Also, the behavior of xTB at CP2K is
doutfull with MD even at ambiant conditions, where the converged volume is
barely larget than at 0K. Depending on EPS_DEFAULT, it can even be smaller
at ambient T. Weird. The behavior of DFTB2 at CP2K is far better.

I found that DFTB+ has other issues. xTB at DFTB+ has no convergence issue,
but the recommended variable-cell optimization algorithm has flaws. The
unit cell and a supercell does NOT always end up with related lattice
parameters. The main issue is that some 90° angles are not preserved with
DFTB+ whereas CP2K does (with no symmetry enforced, obviously). Some
inconsistencies appears in DFTB+ with a lattice dimensions < 10 angstroms
in the unit cell versus > 10 angstroms in the supercell. A proper tight
mesh of k-points does not improve. So I'm afraid that xTB at DFTB+ (or DFTB+,
actually) cannot be a relevant choice for crystal structure predictions,
for instance.

xTB may be unreliable with CP2K and DFTB+, but for the different reasons
above. You can check these weird behaviors with your own crystals of
interest.

Xavier



Le lun. 5 sept. 2022, 3:59 AM, Jürg Hutter <hutter at chem.uzh.ch> a écrit :

> Hi
>
> thank you for testing. Could you send a break down of the energies for the
> LiF molecule for
> the two codes? That might help to recognize the source of the difference.
>
> regards
>
> JH
>
> ________________________________________
> From: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Magnus
> Rahm <magnus at compulartech.com>
> Sent: Monday, September 5, 2022 8:40 AM
> To: cp2k
> Subject: [CP2K:17599] Re: Large discrepancy in xTB results from CP2K vs
> DFTB+
>
> For the record, the problem is the same in CP2K version 2022.1.
>
> On Thursday, September 1, 2022 at 12:48:35 PM UTC+2 Magnus Rahm wrote:
> Dear all,
>
> I want to use CP2K (version 8.2, trying to get a more recent version
> compiled) together with xTB for a crystal containing Li and O. I get
> strange results already for a simple LiO2 crystal:
>
> * There is a very large discrepancy compared to DFTB+ (version 22.1).
> * Mulliken charges tend to be large, meaning that CHECK_ATOMIC_CHARGES
> stops the SCF. If I turn it off, the system tends to converge
> systematically to values just outside the "chemical range". Mulliken
> charges obtained by DFTB+ are significantly smaller (and within "chemical
> range").
> * The energy-volume curve looks strange and very different from DFTB+.
>
> I have tried converging with respect to system size and the EWALD / ALPHA
> and GMAX parameters, but they have only a marginal impact. I have tried
> similar calculations for a number of periodic systems. Sometimes I get
> agreement, sometimes not. I also tried calculations for CO and NO molecules
> which agree perfectly between CP2K and DFTB+, whereas an artificial LiF
> molecule does not.
>
> A perhaps related issue was reported in
> https://groups.google.com/g/cp2k/c/oFwgGcQuySs but the solutions
> suggested there did not solve my problem.
>
> I attach input scripts for CP2K and DFTB+, as well as a figure showing the
> E-V curve for LiO2 obtained with CP2K and DFTB+. I'm new to CP2K, DFTB+ and
> xTB so I suspect I have made some simple mistake, and any advice is
> appreciated.
>
> Kind regards,
> Magnus Rahm
>
>
>
>
>
>
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