# How to calculate friction

Hi all,
I am learning to use lammps to calculate friction. I have problems about how to correctly set the parameters for Fe-Fe interactions. One paper describing the interaction between iron and iron：
the Fe–Fe interactions are modelled using the embedded atom model (EAM) potential. Iron and steel surfaces quickly become oxidised when exposed to air, which significantly reduces the adhesion force between contact surfaces in experimental systems. Therefore, a Lennard-Jones potential was used for Fe–Fe interactions between atoms in opposing slabs n order to mimic the reduced adhesion between oxidised surfaces. The Lennard-Jones parameters, e = 0.02045 eV and r = 3.21 A ° , were used for this interaction.
I tried several ways to set this parameter, but I didn’t get the friction coefficient in the paper.

Two examples:
(1)
Pair_style hybrid eam/fs eam/fs lj/cut 5.4
Pair_coeff * * eam/fs 1 Fe_5.eam.fs Fe NULL
Pair_coeff * * eam/fs 2 Fe_5.eam.fs NULL Fe
Pair_coeff 1 2 lj/cut 0.02045 3.21 5.4

(2)
Pair_style hybrid/overlay eam/fs lj/cut 5.4
Pair_coeff * * eam/fs Fe_5.eam.fs Fe Fe
Pair_coeff 1 2 lj/cut 0.02045 3.21 5.4

I also tried to test with different force fields, including: Fe.eam.fs, Fe_2.eam.fs, Fe_5.eam.fs, Fe_mm.eam.fs, but none of them get the same results as the paper.

Could someone help me to set “Lennard-Jones potential was used for Fe-Fe interactions between atoms in opposing slabs”? thanks very much.

Thanks very much for your help.

jiaoyixiong
Wuhan University of Technology

my input file is :

Hi all,
I am learning to use lammps to calculate friction. I have problems about how to correctly set the parameters for Fe-Fe interactions. One paper describing the interaction between iron and iron：
the Fe–Fe interactions are modelled using the embedded atom model (EAM) potential. Iron and steel surfaces quickly become oxidised when exposed to air, which significantly reduces the adhesion force between contact surfaces in experimental systems. Therefore, a Lennard-Jones potential was used for Fe–Fe interactions between atoms in opposing slabs n order to mimic the reduced adhesion between oxidised surfaces. The Lennard-Jones parameters, e = 0.02045 eV and r = 3.21 A ° , were used for this interaction.
I tried several ways to set this parameter, but I didn’t get the friction coefficient in the paper.

Two examples:
(1)
Pair_style hybrid eam/fs eam/fs lj/cut 5.4
Pair_coeff * * eam/fs 1 Fe_5.eam.fs Fe NULL
Pair_coeff * * eam/fs 2 Fe_5.eam.fs NULL Fe
Pair_coeff 1 2 lj/cut 0.02045 3.21 5.4

this is the correct way to represent the description you quoted from the paper. however, your lennard-jones cutoff seems a bit short. i would have expected at least 8 \AA but 12-14 \AA would be more consistent with typical use of LJ potentials.

(2)
Pair_style hybrid/overlay eam/fs lj/cut 5.4
Pair_coeff * * eam/fs Fe_5.eam.fs Fe Fe
Pair_coeff 1 2 lj/cut 0.02045 3.21 5.4

this is wrong. it adds the LJ terms to the EAM interactions instead of replacing them.

I also tried to test with different force fields, including: Fe.eam.fs, Fe_2.eam.fs, Fe_5.eam.fs, Fe_mm.eam.fs, but none of them get the same results as the paper.

Could someone help me to set “Lennard-Jones potential was used for Fe-Fe interactions between atoms in opposing slabs”? thanks very much.

the friction interaction would be dominated by the LJ interactions in your model, so i am wondering why you worry so much about the EAM part of the model.

axel.