[lammps-users] pair_coeff "incorrect args" with multiple eam pair coefficients


I am trying to model a V-Ti-Cr system with a Finnis Sinclair eam potential that I found In literature. However I noticed that the potential library I am using there’s an error in the Cr-Cr pair coefficients (potential energy tends to minus infinite with decreasing Cr-Cr distance). So, I’m trying to use some Cr-Cr pair_coeff from another potential library. I am trying something like this:

pair_style eam/alloy

pair_coeff 1 1 VTiCrpotential.eam V Ti Cr

pair_coeff 2 2 VTiCrpotential.eam V Ti Cr

pair_coeff 1 2 VTiCrpotential.eam V Ti Cr

pair_coeff 1 3 VTiCrpotential.eam V Ti Cr

pair_coeff 2 3 VTiCrpotential.eam V Ti Cr

pair_coeff 2 2 someCrpotential.eam Cr

However, I get the error “incorrect args for pair coeff” since the first pair_coeff command. It looks like the eam pair_style does not alloy other “I J” elements than “* *”. Is it like this or there is a way in order to do what I am trying to do? Thank you.



The documentation clearly states that with eam/alloy and eam/fs the pair_coeff * * syntax is required. Any mixed parameters must be included in the potential file.


Thank you Axel,

I don’t know I can ask it here. Still, I’d like to know how LAMMPS reads the potential files. I tried to plot the coeffs from the potential file but I don’t get the logic it is written with.

I just see something like, for example:

3 V Ti Cr

10001 0.04999500049995000278 10001 0.00039866013398660134 3.987000000000

23 50.94000 3.03000 bcc





I guess 10001 is the number of coeffs in the same group (V-V, Ti-Ti etc.)

23 is the Z of V, 50.94 its weight and 3.03 its lattice constant.

I tried to plot the first 10001 coeffs and also the second ones but I don’t understand the pattern that LAMMPS uses to read such files and rearrange the potentials.

e.g. I’d like to find the (I guess) 10001 coeffs of the Cr-Cr interaction to mirror them about the x axis (assuming x-axis is the distance between the two particles). Thank you.



The different EAM file formats are described in detail the manual (and the corresponding publications). Also there are programs in the tools folder that show how to write such potential files.

There is no need to guess.

Transplanting a different potential into an existing potential like you describe is rarely a good idea. It will result in inconsistent interactions. More likely there is an error in how your original potential was created or parameterized. That can only be corrected by identifying and correcting the faulty step or by looking up and using an alternate parameter set or potential file.