[lammps-users] Need help implementing mixed (cross) EAM-FS potential for Fe-Cr in LAMMPS

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1

Hello,

I am trying to write a Dynamo setfl file containing three EAM Finnis-Sinclair (FS) type potentials for an Fe-Cr alloy. The three potentials are for Fe-Fe, Cr-Cr, and the Fe-Cr cross-interactions.

I have a specific potential form for Fe-Cr and as such do Not want LAMMPS to mix the other two potentials for me to create a third Fe-Cr cross-potential.

I can get LAMMPS to work with just the Fe-Fe potential, but I get errors when introducing the cross-potential (for Fe-Cr). Looking through messages in the LAMMPS mail list archives, I believe that I should accomplish this by placing all three potential in the same Dynamo setfl file.

Looking over the “read_file” function in “pair_eam_fs.cpp” file of the LAMMPS source code, I believe I understand how to implement EAM-FS potentials for Fe-Fe and Cr-Cr interactions, but I don’t know how to do it for the Fe-Cr cross potential.

If I’m right, the Dynamo setfl file format should follow this structure (Let i = for element i):

Line 6: [Atomic Number(i)] [Atomic Mass(i)] [Lattice Constant(i)] [Lattice Type(i)]
Line 7 to A: [Embedding Function(i) = F(rho(i))]
Line A+1 to B: [Electron Density(i) = rho(i)]
…
Line B+1: [Atomic Number(j)] [Atomic Mass(j)] [Lattice Constant(j)] [Lattice Type(j)]
Line B+2 to C: [Embedding Function(j) = F(rho(j))]
Line C+1: [Electron Density(j) = rho(j)]
…
(Continue on for however many elements you have)
…
Line D: [Pair Potential between i and i]
…
Line E: [Pair Potential between i and j]
…
Line F: [Pair Potential between i and k]
…
(Continue on for however many elements you have)
…
Line G: [Pair Potential between n and n]

If I am right, there seems to be nowhere to specify the electron density function for the cross potential. If I am wrong…I’d love to know where I’m wrong.

Any help you can offer me is greatly appreciated!

Cheers,
-Brian Frisbie

2

If you use a setfl file, LAMMPS does no mixing, since the cross
interactions are in the file. The pair_style eam doc page
describes the format of the setfl file. That and the source
code are all there is.

Steve

3

Thanks for the reference,

I’ve read through that document as well and I believe it agrees with my description of the Dynamo setfl file format in my prior email.

My Problem: I know it is possible to list/specify cross interactions for the Pair Potential, however it does NOT appear possible to list/specify cross interaction for the Density Function, rho®.

I need to specify cross interactions for the Density Function, rho®. Can I do this using LAMMPS? It does not seem possible using the setfl format. I’ve copied the relevant section from the “pair_style eam doc” page to the end of this email for reference.

Thank you again for your suggestions and for getting back to me so quickly.

Regards,
-Brian Frisbie

4

I think you want the Finnis-Sinclair format, which is eam/fs in LAMMPS.
It allows a pairwise rho() definition. The eal/alloy (setfl) format
use an element-wise rho().

Steve

5

You’re right! That solves my problem.

Thank you so much for your help!

Cheers,
-Brian Frisbie