[CP2K-user] [CP2K:17609] Re: Large discrepancy in xTB results from CP2K vs DFTB+
Magnus Rahm
magnus at compulartech.com
Mon Sep 5 13:19:28 UTC 2022
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
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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