[CP2K-user] [CP2K:20027] Matsui-Akaogi Force Field for bulk TiO2

[Léon Luntadila Lufungula]

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>:
[image: MA cell params.png]

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]
[image: MA definition.png]
[image: MA params.png]
- 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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