Double counting in combined sw and tersoff potential

[merlin021]

Hi, all,
  I have a Silicon structure with Silica (amorphous) inside. I want to use Si-sw potential for Silicon atoms (all Si atoms including those in Silica) and SiO2-tersoff potential for Silica (only the part for Si-O interaction). Now if I combine both potentials in the phonon transmission calculation, the Silicon interaction will be double counted by both sw ans tersoff, is there a way of turning off the Silicon interaction in tersoff potential so that only sw potential works for it ?
  I appreciate any suggestions from you, thanks a lot !

[Ulrik G. Vej-Hansen]

Yes, it should be explained in our tutorial on combining and modifying classical potentials: https://docs.quantumwise.com/tutorials/combining_potentials/combining_potentials.html

Just let me know if anything is unclear.

[merlin021]

Hi, Sir,
  Thank you very much for your prompt reply.
   Unfortunately I failed to find something pertinent to my problem above there (maybe I missed it, though).
   Like what I did in LAMMPS, I simply set parameters for si-si-si interaction in SiO-tersoff to zero, then I get no double-counting for Si interaction, and only Si-sw works for it. I just want to do the same thing here in QW otherwise I get double-counting.

[Ulrik G. Vej-Hansen]

I think this section describes almost exactly what you want to do, but maybe I misunderstand your problem: https://docs.quantumwise.com/tutorials/combining_potentials/combining_potentials.html#combining-a-tersoff-and-a-lennard-jones-potential

You should be able to follow that procedure, and then simply omit the Si-Si terms from the Tersoff potential.

[merlin021]

I think this section describes almost exactly what you want to do, but maybe I misunderstand your problem: https://docs.quantumwise.com/tutorials/combining_potentials/combining_potentials.html#combining-a-tersoff-and-a-lennard-jones-potential

You should be able to follow that procedure, and then simply omit the Si-Si terms from the Tersoff potential.

Dear Sir, Would you please tell how to omit the Si-Si terms in Tersoff while keeping that in SW potential ?
Thank you very much.

[Julian Schneider]

You can switch off the Si-Si interaction in  the Tersoff potential, but I would not recommend it, because, since the Tersoff potential is a manybody potential, it is not really made for disentangling the two and three body parts, meaning that removing the Si-Si interactions might have unpredictable consequences on the Si-O interactions.
Instead I would suggest to use the Tersoff Si-O potential for the entire system, as the Si-Si interaction in that potential are very similar to the original Tersoff potential for Silicon (Tersoff_Si_1988), with only slight modifications to few parameters (see the paper http://www.sciencedirect.com/science/article/pii/S0927025606002023).
You could test the potential for a silicon system alone to see if it reproduces the desired properties for silicon sufficiently well.

If you really want to use the SW potential for Si-Si, you have to follow the genral procedure to modify potentials, as outlined in the tutorial above, in your case you specifically have to remove the TersoffSingleTypePotential for Si and instead of the TersoffMixitPotential for Si-O you have to use the TersoffTriplePotential and specifiy all Si-O pair parameters manually using the combination rules given in the paper.
See also https://docs.quantumwise.com/manuals/Types/TersoffTriplePotential/TersoffTriplePotential.html#tersofftriplepotential-c  for more details.
Finally, the SW potential classes need to be added to the potential set.