[lammps-users] discussion on the contact

Dear everyone,

Now I am using large-scale molecular dynamics simulations with tersoff potential by Lammps. In our MD studies, we use flat diamond sample and spherical tip made of diamond-like carbon which is rigid and fractal rough.

Simulations of normal loading are performed at 300K by alternating loading and holding phases. In the loading phase, the tip is displaced toward the sample in the increments of 0.1 Å,and the holding phases last 10 ps.The process is performed in the LAMMPS by the following commands:
velocity indenter set 0 0 0 units box
fix 6 indenter setforce 0.0 0.0 0.0
displace_atoms indenter move 0 0 -0.1 units box
run 5000

Now the problem is that the tip cannot be impressed into the sample, but remain staying on the sample surface. My input file is as follows. Can you tell me whether the errors in it and how to perform this process by LAMMPS. Thanks very much.

3d contact simulation

units metal
boundary p p s
atom_style atomic
neighbor 2.0 bin
neigh_modify delay 5

create geometry

read_data data.indenter
group indenter type 1
lattice diamond 3.5667
region sample block 0 10 0 10 0 8
create_atoms 1 region sample

Tersoff potentials

pair_style tersoff
pair_coeff * * SiC.tersoff C C C
neigh_modify exclude type 1 1
neigh_modify exclude type 2 2

define groups

region lower block INF INF INF INF INF 2
group sample region sample
group lower region lower
group mobile subtract sample lower
mass 1 12
mass 2 12
mass 3 12
set group indenter type 1
set group lower type 2
set group newton type 3

temp controllers

compute new1 mobile temp
velocity mobile create 300.0 3618360 temp new1 dist gaussian

equilibrate

fix 1 mobile nve
fix 2 lower setforce 0.0 0.0 0.0
fix 3 indenter rigid single
fix 4 mobile temp/rescale 5 300 300 2.0 1.0
fix_modify 4 temp new1

contact

dump 1 all atom 1000 test.lammpstrj
dump_modify 1 scale no
timestep 0.002
run 5000
thermo 50
thermo_style custom step c_new1 etotal vol
log log.test
run 5000

displace_atoms indenter move 0 0 -0.1 units box
velocity indenter set 0 0 0 units box
fix 6 indenter setforce 0.0 0.0 0.0
run 5000

displace_atoms indenter move 0 0 -0.1 units box
velocity indenter set 0 0 0 units box
fix 6 indenter setforce 0.0 0.0 0.0
run 5000

Best regards.

Haidong Lei