[CP2K-user] [CP2K:20031] Matsui-Akaogi Force Field for bulk TiO2

[Krack Matthias]

Hi Léon

I think there is no need to use GENPOT, because that nonbonded force field contribution is implemented in CP2K as BUCKMORSE<https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/MM/FORCEFIELD/NONBONDED/BUCKMORSE.html>. There are often typos in papers with force field parameters and it is advisable to cross-check the parameter tables for the same force field in different papers for consistency.
Small differences to other codes can have many reasons: splining quality, single- vs double-precision, SPME mesh size, type and threshold values of the employed optimizer etc.

Best

Matthias

From: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Léon Luntadila Lufungula <Leon.luntadilalufungula at uantwerpen.be>
Date: Thursday, 14 March 2024 at 11:48
To: cp2k <cp2k at googlegroups.com>
Subject: Re: [CP2K:20027] Matsui-Akaogi Force Field for bulk TiO2
Hi Matthias,

Thanks for pointing this out for me, I've adjusted the input accordingly and now the calculation seems to work fine. I've increased GMAX to 2 points per angstrom and also used MULTIPLE_UNIT_CELL 8 8 3 (each cell vector should thus be around 30 Angstrom in length), because I read that the cell edge should be at least twice the size of RCUT (i.e., > 24 Angstrom). I tried several versions of the Matsui-Akaogi Force Field to try and get decent values for the cell parameters after a cell optimization as was shown in their original paper<https://www.tandfonline.com/doi/abs/10.1080/08927029108022432>:
[cid:c58cf394-bb9d-474d-a488-46c037a89594]

However, no matter which definition I try, I always get cell parameters which are quite a bit larger than those reported and I can't seem to get the original FF definition to work with GENPOT. The parameter sets I tried are attached and labeled as follows:
- Matsui-Akaogi (MA)<https://www.tandfonline.com/doi/abs/10.1080/08927029108022432>: original definition with GENPOT [CPASSERT failed]
[cid:a71cb60c-82b7-44d6-b297-a7d2c27cffe6]
[cid:069c437e-4f67-4243-9be3-dc6c4a21c4a7]
- Naicker (N)<https://pubs.acs.org/doi/10.1021/jp050963q>: seemingly unaltered WILLIAMS version of the original MA definition, although I'm not 100% sure how to convert the original function into a WILLIAMS type definition. But f(Bi+Bj) does coincide with rho_ij and Cij coincides with (Ci+Cj) [a = 3.8865, c = 10.0187]
- Predota (P)<https://pubs.acs.org/doi/10.1021/jp037197c>: slightly altered version to better simulate TiO2 surfaces [a = 3.8865, c = 10.0187]
- Alimommadi (A)<https://pubs.acs.org/doi/10.1021/jp207272e>: parameters should be the same as Predota but are given in eV instead of kcalmol, but give slightly different result [a = 3.886, c = 10.0177]

Do you know why this could be? Is there still something wrong with my settings perhaps or should I just accept that the result might not be ideal? Also, I don't see what I'm doing wrong with the GENPOT definition of the original MA FF, so any help would be greatly appreciated!

Kind regards,
Léon
On Wednesday 13 March 2024 at 18:26:08 UTC+1 Krack Matthias wrote:
Hi Léon

your force field definition is incorrect. The unit for the C parameters should be [kcalmol*angstrom^-6] instead of [kcalmol*angstrom^-1] and the B parameter for Ti-O should be something like 5.15 rather than 0.00515. The simulation cell is also too small for the given RCUT value of 12, but that shouldn’t matter.

HTH

Matthias

From: cp... at googlegroups.com <cp... at googlegroups.com> on behalf of Léon Luntadila Lufungula <Leon.luntad... at uantwerpen.be>
Date: Wednesday, 13 March 2024 at 12:20
To: cp2k <cp... at googlegroups.com>
Subject: [CP2K:20022] Matsui-Akaogi Force Field for bulk TiO2
Dear all,

I want to test a force field for TiO2 so that I can use it for pre-equilibrating my system for AIMD calculations. However, I have no experience in using force fields and I'm having trouble implementing the FF calculations. I took the FF parameters from a paper from Naicker et al<https://pubs.acs.org/doi/10.1021/jp050963q> and tried to calculate the energy of the anatase crystal structure, before going on to a cell optimization of the structure to compare to DFT.

However, when I try to run the calculation I get an error indicating "GEOMETRY wrong or EMAX_SPLINE too small!", however, the geometry is definitely correct. So I tried increasing EMAX_SPLINE to 1E2 and then got "SPLINE_INFO| Number of points:  2346797 obtained accuracy 0.953210E-07. MM SPLINE: no convergence on required accuracy (adjust EPS_SPLINE and rerun)". I then tried to increase EPS_SPLINE to 1E-6 and the energy calculation ran without a problem. I tried to run a CELL_OPT with these settings but the optimization seems to be going nowhere... (see attached input and output files) Can anyone tell me what I'm doing wrong? Is it perhaps the definition of the forcefield?

Kind regards,
Léon
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