[lammps-users] Argon simulation with LJ potential

Hello all,

I am trying to simulate an object/obstacle moving through liquid argon. I am using LJ parameters given for argon with real units, but the atoms are behaving like solid and not liquid even above the melting point of argon. My input script is as follows:

Thanks,

Bhavin N. Vadgama

# 3d LJ obstacle moving in liquid Argon

units real
dimension 3
boundary p s p

atom_style atomic
neighbor 2.0 bin
neigh_modify delay 5

# create geometry

lattice fcc 5.25
region box block 0 20 0 15 -5 5
create_box 4 box
create_atoms 1 box

mass 1 39.95
mass 2 39.95
mass 3 39.95
mass 4 39.95

# LJ potentials

pair_style lj/cut 8.5
pair_coeff * * 0.238 3.405 8.5

# define groups

region 1 block INF INF INF 1.25 INF INF
group lower region 1

region 2 block INF INF 13.75 INF INF INF
group upper region 2

region obstacle cylinder y 10 0 2 5 10
group forced region obstacle

group boundary union lower upper
group flow subtract all boundary forced

set group lower type 2
set group upper type 3
set group forced type 4

# initial velocities

compute mobile flow temp
velocity f low create 85.0 482748 temp mobile
fix 1 all nve
fix 2 flow temp/rescale 200 85.0 85.0 0.02 1.0
fix_modify 2 temp mobile

# Fixed Boundary

velocity boundary set 0.0 0.0 0.0
fix 3 lower setforce 0.0 0.0 0.0
fix 4 upper setforce 0.0 0.0 0.0

# Moving obstacle

velocity forced set 0.0 0.0 0.0
fix 7 forced rigid single
fix 8 forced aveforce 100.0 0.0 0.0

# Run

timestep 0.003
thermo 1000
thermo_modify temp mobile

dump 1 all atom 100 dump.obs3dreal

run 30000

I would simulate bulk LJ w/out the obstacle in both
lj and real units and see if you can get the same answer.
If you do the units conversion properly you should get
exactly the same answer.

Steve

Hello Steve,

As per your suggestion, I tried to simulate the simpler couette flow example given in lammps in LJ units (as is) and real units. The conversion of LJ to real units is as per attached excel sheet. I am still not getting the same results. I am not very sure about my time/timestep conversions. It would be great if you can look at the excel file and my input script.

Thanks for your help.
Bhavin N. Vadgama

LJ_Conversion.xls (13.1 KB)