[CP2K-user] [CP2K:17614] Re: Large discrepancy in xTB results from CP2K vs DFTB+
Jürg Hutter
hutter at chem.uzh.ch
Tue Sep 6 09:37:10 UTC 2022
Hi
I think I found the problem. This is in fact a bug in CP2K and is related to the damping of
the "short range" part of the Coulomb term. As mentioned before this short range part
is not short range at all, even diverging in periodic systems. We use a damping function
for this term and the radius is taken from the range of the basis function on each atom.
The bug is now, that this range is not a constant but depends on EPS_DEFAULT.
I will work on a solution, but at least the default settings will cause considerable changes
in the energies of periodic systems.
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 3:19 PM
To: cp2k
Subject: Re: [CP2K:17609] Re: Large discrepancy in xTB results from CP2K vs DFTB+
Hi,
Thank you for valuable input! Here's a breakdown of energies for a periodic LiO2 system (where CP2K and DFTB+ disagree).
CP2K:
Core Hamiltonian energy: -609.45757320827579
Repulsive potential energy: 2.86335541921533
Electronic energy: -65.73940900376786
DFTB3 3rd order energy: 9.00274299587460
Dispersion energy: -2.00065978643714
Correction for halogen bonding: 0.00000000000000
Total energy: -665.33154358339084
outer SCF iter = 1 RMS gradient = 0.49E-06 energy = -665.3315435834
outer SCF loop converged in 1 iterations or 10 steps
And the same system with DFTB+ (I don't know this is the best breakdown I can get from DFTB+? This info is from detailed.out.):
Fermi level: -0.1574062769 H -4.2832 eV
Band energy: -254.9890864567 H -6938.6061 eV
TS: 0.0000000000 H 0.0000 eV
Band free energy (E-TS): -254.9890864567 H -6938.6061 eV
Extrapolated E(0K): -254.9890864567 H -6938.6061 eV
Input / Output electrons (q): 864.0000000000 864.0000000000
Energy H0: -610.3586854777 H -16608.7049 eV
Energy SCC: 13.1915555608 H 358.9605 eV
Total Electronic energy: -597.1671299169 H -16249.7444 eV
Repulsive energy: 0.0000000000 H 0.0000 eV
Total energy: -597.1671299169 H -16249.7444 eV
Extrapolated to 0: -597.1671299169 H -16249.7444 eV
Total Mermin free energy: -597.1671299169 H -16249.7444 eV
Force related energy: -597.1671299169 H -16249.7444 eV
----------------------------------------------------------------------------------------------------------------
For reference, here are the equivalent breakdowns for the LiF molecule, where the total energies do match quite well.
CP2K:
Core Hamiltonian energy: -5.57594122418510
Repulsive potential energy: 0.00036401843654
Electronic energy: 0.08477836575096
DFTB3 3rd order energy: -0.00385103760005
Dispersion energy: -0.00008325087778
Correction for halogen bonding: 0.00000000000000
Total energy: -5.49473312847544
outer SCF iter = 1 RMS gradient = 0.12E-06 energy = -5.4947331285
outer SCF loop converged in 1 iterations or 25 steps
DFTB+
Fermi level: -0.3434874008 H -9.3468 eV
Band energy: -3.7493389034 H -102.0247 eV
TS: 0.0000000000 H 0.0000 eV
Band free energy (E-TS): -3.7493389034 H -102.0247 eV
Extrapolated E(0K): -3.7493389034 H -102.0247 eV
Input / Output electrons (q): 8.0000000444 8.0000000000
Energy H0: -5.5743451431 H -151.6856 eV
Energy SCC: 0.0807122067 H 2.1963 eV
Total Electronic energy: -5.4936329365 H -149.4894 eV
Repulsive energy: 0.0000000000 H 0.0000 eV
Total energy: -5.4936329365 H -149.4894 eV
Extrapolated to 0: -5.4936329365 H -149.4894 eV
Total Mermin free energy: -5.4936329365 H -149.4894 eV
Force related energy: -5.4936329365 H -149.4894 eV
----------------------------------------------------------------------------------------------------------------
> 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.
Thank you for sharing this info! I tried a series of calculations with LiO2 using varying values of EPS_DEFAULT (using default EPS_SCF) and found the same effect; no convergence with EPS_DEFAULT (or perhaps unreasonably slow convergence). I attach a figure showing these results, including the energy broken down into the different parts as specified in the CP2K output. Note the energy scale, the changes with EPS_DEFAULT are really quite substantial. In the LiF (non-PBC) case, the corresponding curves look completely flat on the same scale. I don't know what to make of this result, but perhaps someone else does?
Magnus
[X]
On Monday, September 5, 2022 at 12:18:26 PM UTC+2 jazz... at gmail.com wrote:
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 <hut... 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: cp... at googlegroups.com <cp... at googlegroups.com> on behalf of Magnus Rahm <mag... 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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