Questions about potentials

[Fishpack]

I want to calculate the phonon dispersion curve of graphene with O atoms.

That means I need a potential that can explain C-O bonding.

In ATK, some reaxFF support the C-O bonding.

But, I obtained the incorrect phonon dispersion using reaxFF.

i) If there are possible to obtain the phonon dispersion using reaxFF, please let me know.

ii) Is it possible to use airebo potential? Actually, I know the parameter about the airebo potential.

iii) If possible, I want you to recommend some potentials that can support C-O bonding.

Thank you.

[Julian Schneider]

i) If there are possible to obtain the phonon dispersion using reaxFF, please let me know.

In principle there should be no problem calculating the Phonon dispersion with ReaxFF, but phonons are probably not the primary type of property that ReaxFF potentials are fitted to.
Moreover, many ReaxFF potentials only give good results for the context they are parameterized for.  E.g. a potential that is parameterized primarily for SP3 Carbon atoms, might not give good results when applied to graphene. You should check the publication for that potential to find out if its the purpose it has been designed for, matches your application.

ii) Is it possible to use airebo potential? Actually, I know the parameter about the airebo potential.

Unfortunately Airebo potentials are currently not implemented in ATK. I'll check if there is a workaround, but I don't think so.

iii) If possible, I want you to recommend some potentials that can support C-O bonding.

I'll check if I can find a suitable potential.

[Fishpack]

I've got a question about changing the C atom mass with maintaining the bonds.

Can you tell me the way? Actually, I've changed the mass of C atoms through the C14 nanotube case.

However, I think the property of bonds between the  C atoms  are missing because the phonon dispersion has no branch of LA, TA, ZA.

I saw only horizontal lines in my phonon dispersion.

Thank you!

[Julian Schneider]

Regarding the potentials for C-O systems:
Currently, we only provide ReaxFF potentials, for such systems. The first one in the list sounds as if it may be close to what you are looking for ("ReaxFF reactive force field for molecular dynamics simulations of hydrocarbon oxidation"), however, you'd need to check if it works for graphene, as well. And it might be that it hasn't been parameterized to reproduce the dynamical matrix.
We are working on implementing REBO potentials at the moment, I'll check whether that can be extended to AIREBO potentials.

Bonds and potentials only depend on the elements, they are not influenced by the changing the masses of an element.

[Fishpack]

Thank you for reply!

Actually, we want to calculate the phonon dispersion of graphene with Pt and O atoms.
(you may notify our purpose if you see my previous posts)

I have tried some kinds of potentials that has C, O and Pt atoms that includes reaxFF.

But, they didn't work.

So, we just want to use C tersoff potential with changing the mass of some atoms to the mass of Pt or O atoms to know the tendency.

That is the reason why I ask some questions of changing mass of some atoms.

1) Is it possible to calculate the system what I'm mentioned? (C tersoff with changing the mass of some atoms)

2) Can the mass change affect the phonon dispersion?

[Fishpack]

Moreover, I've a question about the phonon dispersion curve.

The attached file shows the phonon dispersion curve based on the atom structure (right side).

The ZA phonon branch has negative energy.

I wonder that it has negative energy although the optimization was performed.

Do you have any comments for that?

The system is consisted with C atoms. The C tersoff potential was used for the calculation of phonon dispersion and the optimization.

[Jess Wellendorff]

Are you absolutely certain that forces were sufficiently minimized in geometry optimization?

[Fishpack]

You mean that "the sufficiently minimized" is the lowest energy of the system as possible as we can?

[Jess Wellendorff]

By sufficient I mean that residual forces should be sufficiently small for a phonon analysis. Standard force minimization thresholds are not always sufficient for subsequent phonon calculations.