Coul/Debye with different kappa

Hello lammps community,

I am trying to use Coul/Debye potential with different kappa parameters for different type of pairs. I tried to look at couple of options

It looks like as it is not a pair coefficient, it can’t be given using pair_coeff.

The same issue is occurring if one wish to use Hybrid pair style and use coul/debye as multiple sub-styles. Then it is hard to differentiate for which type of pairs which coul/debye will be applied.

Fix adapt only has scale factor change option but not for kappa.

Can someone suggest possible way to do this.

Presently I am working on two options …

  1. Try to add it in fix adapt for coul/debye pair_style
  2. How about if I just save pair_style coul/debye with different names (and function names changed inside code too…) this way I think I can use pair_style hybrid.

How correct it is to adopt second option.

Thanks

The documentation is a bit unclear for me as well but judging from the source (pair_coul_debye.cpp) the first argument to pair_coeff after the types is actually kappa and the second argument is the cutoff, so you can in fact do

pair_coeff 1 1 <some kappa for pairs of types 1>

pair_coeff 1 2 <other kappa for pairs of types 1 and 2>

etc. Is that what you want to achieve?

Hi Stefan,

Actually this would have helped but for coul/debye pair style, kappa I.e. inverse debye length comes with pair style command arguments and pair_coeff is optional to set only cutoff.

Hi Stefan,

Actually this would have helped but for coul/debye pair style, kappa I.e. inverse debye length comes with pair style command arguments and pair_coeff is optional to set only cutoff.

For the debye model the screening of electrostatic interactions is a function of the overall material and thus assumed to be constant. Hence it is specified with the pair style and not as pairwise parameter.

Axel

Yes Axel,
but if one makes it variable then it may also serve as …de brogali wavelength dedicated to quantum shielding Which may be different for different species in system.

Hello lammps community,

I am trying to use Coul/Debye potential with different kappa parameters for different type of pairs. I tried to look at couple of options

It looks like as it is not a pair coefficient, it can’t be given using pair_coeff.

The same issue is occurring if one wish to use Hybrid pair style and use coul/debye as multiple sub-styles. Then it is hard to differentiate for which type of pairs which coul/debye will be applied.

Why is that? You have to mention explicitly to which pair style instance which pair_coeff statement is to be applied, for all others you can use none.

Fix adapt only has scale factor change option but not for kappa.

How should fix adapt help with this?

Can someone suggest possible way to do this.

What you describe is a completely different model (and I wouldn’t know what to call it or how to justify it), so the clean approach would be to implement a new pair style.

Axel

Thanks Axel,

You may have a look on model here:

http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.41.1379

Thanks Axel,

You may have a look on model here:

http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.41.1379

​And this can be achieved in lammps using Hybrid or hybrid/overlay pair_style.

The point remains is: if it is possible to put kappa (inverse debye length) in
pair_coeff than in pair_style, then it can be changed for different pairs depending on
mass of species …to use it as de-brogali wavelenth.

Like I already wrote in my response, what you describe is a different model that requires writing a new pair style or using pair hybrid with as many instances of coul/debye as you have different pairs.

Simply teaching pair_coeff to accept a additional parameter doesn’t change the implementation, which only uses a single variable for kappa. Thus each subsequent pair_coeff statement would simply overwrite the previous setting.

Axel