Dear steve,
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.test
group indenter type 1
lattice diamond 3.5667
region sample block 0 10 0 10 0 8
create_atoms 1 region sample
#potentials
pair_style airebo 3.0
pair_coeff * * CH.airebo 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 mobile 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
displace_atoms indenter move 0 0 -19 units box
velocity indenter set 0 0 0 units box
fix 6 indenter setforce 0.0 0.0 0.0
thermo 50
thermo_style custom step c_new1 etotal f_6[3] vol
log log.test
run 5000
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
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
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
…
Best regards.
Haidong Lei