# Lubrication model

Hello every one,

I’m trying to develop a model for lubrication where I have Al in direct contact with C9H20.
I have some problem with defining the pair_style, I would appreciate if anyone can help me with this.

Currently I have this model:

units real
atom_style full
bond_style hybrid harmonic
angle_style hybrid harmonic
dihedral_style hybrid fourier
improper_style hybrid cvff
pair_style hybrid lj/cut/coul/long 10.0 eam
kspace_style pppm 0.0001

pair_coeff * * eam Al1.eam.fs
pair_coeff 4 4 lj/cut/coul/long 0.0150 2.59964245953
pair_coeff 2 2 lj/cut/coul/long 0.0860 3.39966950842
pair_coeff 3 3 lj/cut/coul/long 0.1094 3.39966950842

Problem here is since my system doesn’t have any charge, I should not use the kspace, but if I don’t use it, it gives me error that I haven’t defined kspace which make sense for the C9H20 part.

I was wondering if you can help me figure out how or what combination of pair_style hybrid I have to define in order to solve this problem.

Ale

If you have no charge, you also shouldn’t be using lj/cut/coul/long.

Just use lj/cut with your pair hybrid.

Steve

Dear Steve,

Thanks for your response. I figured that out in the weekend I forgot to take down the question.

But now I have another problem, which I really appreciate if you can help me figure that out. I’m modeling a lubricant on top of an Al block, and I’m applying two forces:

1- velocity applied to the block
2- gravity

I observed two behavior which are not correct, so I was wondering if you can help me out understand those:

1- When I apply velocity to the lower layers of the block, just that region moves and the rest of the block is not moving. The only thing that I can think of is the velocity that I apply causes much higher forces than available lj potential forces which overcome the lj force, so I’m trying to lower the velocity but so far, I didnt see any change.

2- The gravity force also makes the Al block gets more packed, I think it causes by the same reason, high forces.

Sorry this questions are not directly related to the LAMMPS but since I’m a newbie in LAMMPS I would really appreciate your help.

Here is my code by far:

# ------------------------------- Initialization Section --------------------

units real
atom_style full
bond_style hybrid harmonic
angle_style hybrid harmonic
dihedral_style hybrid fourier
improper_style hybrid cvff
pair_style hybrid eam/alloy lj/cut 10.0

# ------------------------------- Settings Section --------------------------

pair_coeff * * eam/alloy Al99.eam.alloy Al Al NULL
pair_coeff 3 3 lj/cut 0.0150 2.59964245953
pair_coeff 2 2 lj/cut 0.0860 3.39966950842

pair_coeff 1 3 lj/cut 0.0150 2.59964245953
pair_coeff 1 2 lj/cut 0.0860 3.39966950842

bond_coeff 1 harmonic 478.4 1.3870
bond_coeff 2 harmonic 344.3 1.0870

angle_coeff 1 harmonic 67.18 119.97
angle_coeff 2 harmonic 48.46 120.01

dihedral_coeff 1 fourier 1 3.625 2 180.0

improper_coeff 1 cvff 1.1 -1 2

region lo-fixed block INF INF INF INF INF 8.1
region lo-slab block INF INF INF INF INF 24.3
region above-lo block INF INF INF INF INF 24.3 side out

region hi-fixed block INF INF INF INF 46.2 INF
region hi-slab block INF INF INF INF 30 INF
region below-hi block INF INF INF INF 30 INF side out

group lo-slab region lo-slab
group above-lo region above-lo
group hi region hi-slab
group lo-fixed region lo-fixed
group hi-fixed region hi-fixed
group boundary union lo-fixed hi-fixed
group mobile subtract all boundary

# ------------------------------- Run Section -------------------------------

Anytime you are applying external forces or imposing

a velocity, you need to make sure their magnitude

is reasonable for the unit system you are using.

Using gravity = 1.0 for real units is unlikely to be

correct. Real units is atoms and atoms don’t feel

gravity, since gravity is orders of magnitude too

small a force at the atomic scale. So if you

are applying some arbitrarily large force as “gravity”

then a system may densify.

Steve