creating dlc

[Ottman_Tertuliano1]

Hello,

I am using the tersoff potential in order to create a dlc tip; essentially, i am following this recipe by Schall:

1. Set the desired final density by scaling the atomic and box coordinates to the appropriate
   values.
2. Thermalize at 300K and Heat to 8000K in NVE. I like to actually melt the diamond.
3. Equilibrate in NVE @ 8000K.
4. Quench to 300K. Quench times vary but I usually use only a few picoseconds.
5. Equilibrate at 300K in NPT to relieve any stresses and/or minimize energy, depending on your
   final objective.

I created a sphere randomly populated with carbon atoms based on a .4atom/angs^3 density(cubic cell diamond density) and used fix temp/rescale for everything up to the last step. the simulation runs at about 1000000 bars through out(is that sort of pressure normal?). im having issues equibrating at 300k in npt after quenching. the system temperature increases drastically.

any help would be appreciated
thanks,

here is my script:

#set dimension boundaries
dimension 3
boundary p p p
units metal

#set atom style
atom_style atomic
neighbor 2.0 nsq

#create simulation box, 10angstrom cube centered about orgin
region exper block -20.0 20.0 -20.0 20.0 -20.0 20.0 units box
create_box 1 exper

#create sphere with 5 angstrom radius
region tip_sphere sphere 0.0 0.0 0.0 10.0 units box

create_atoms 1 random 1658 879456 tip_sphere units box

#cutoff value, 3.0 is a scale. Multiplied by 3.2Angstroms from AIREBO file.
pair_style tersoff
pair_coeff * * SiC.tersoff C

#atomic mass of carbon
mass 1 12.0107

define groups

group tip region tip_sphere
set group tip type 1

#thermalize to 300k with nve
fix bound tip indent 300.0 sphere 0.0 0.0 0.0 10 side in units box

compute init_temp tip temp
velocity tip create 300.0 887799 temp init_temp

fix 1 tip temp/rescale 1 300 300 .1 1
fix 2 all nve
timestep .001

thermo 100
thermo_style custom step temp etotal pe press

dump 1 all atom 100 dump.ind_step1
run 2500

#heat up to 8000
unfix 1
fix 3 tip temp/rescale 1 300 8000 .5 1
fix 4 tip spring tether 50.0 0.0 0.0 0.0 0.0

dump 2 tip atom 100 dump.ind_step2
run 6000

#equilibrate at 8000k
unfix 3
fix 5 tip temp/rescale 1 8000 8000 1 1

dump 3 all atom 100 dump.ind_step3
run 3000

#compute info for atoms in spherical tip

compute count_loc all coord/atom 1.77
compute count all reduce sum c_count_loc
#variable bond_count equal “c_count / 2”

#variable tot_energy equal thermo etotal
#variable pot_energy equal thermo pe

#fix outpress all print 100 “{ttime} {otemp} {count_limit} {bond_count} {tot_energy} {pot_energy}” file tip_info.txt

#quench to 300k
unfix 5
fix 7 tip temp/rescale 1 8000 300 .5 1

dump 4 all atom 100 dump.ind_step4
run 3000

#equilibrate at 300k

unfix 2
unfix 7

fix 8 all npt temp 300 300 100 iso 1000000 1 1000
dump 5 all atom 100 dump.ind_step5
run 6000

[Ottman_Tertuliano]

Hello,

I am using the tersoff potential in order to create a dlc tip; essentially, i am following this recipe by Schall:

1. Set the desired final density by scaling the atomic and box coordinates to the appropriate
   values.
2. Thermalize at 300K and Heat to 8000K in NVE. I like to actually melt the diamond.
3. Equilibrate in NVE @ 8000K.
4. Quench to 300K. Quench times vary but I usually use only a few picoseconds.
5. Equilibrate at 300K in NPT to relieve any stresses and/or minimize energy, depending on your
   final objective.

I created a sphere randomly populated with carbon atoms based on a .4atom/angs^3 density(cubic cell diamond density) and used fix temp/rescale for everything up to the last step. the simulation runs at about 1000000 bars through out(is that sort of pressure normal?). im having issues equibrating at 300k in npt after quenching. the system temperature increases drastically.

any help would be appreciated
thanks,

here is my script:

#set dimension boundaries
dimension 3
boundary p p p
units metal

#set atom style
atom_style atomic
neighbor 2.0 nsq

#create simulation box, 10angstrom cube centered about orgin
region exper block -20.0 20.0 -20.0 20.0 -20.0 20.0 units box
create_box 1 exper

#create sphere with 5 angstrom radius
region tip_sphere sphere 0.0 0.0 0.0 10.0 units box

create_atoms 1 random 1658 879456 tip_sphere units box

#cutoff value, 3.0 is a scale. Multiplied by 3.2Angstroms from AIREBO file.
pair_style tersoff
pair_coeff * * SiC.tersoff C

#atomic mass of carbon
mass 1 12.0107

define groups

group tip region tip_sphere
set group tip type 1

#thermalize to 300k with nve
fix bound tip indent 300.0 sphere 0.0 0.0 0.0 10 side in units box

compute init_temp tip temp
velocity tip create 300.0 887799 temp init_temp

fix 1 tip temp/rescale 1 300 300 .1 1
fix 2 all nve
timestep .001

thermo 100
thermo_style custom step temp etotal pe press

dump 1 all atom 100 dump.ind_step1
run 2500

#heat up to 8000
unfix 1
fix 3 tip temp/rescale 1 300 8000 .5 1
fix 4 tip spring tether 50.0 0.0 0.0 0.0 0.0

dump 2 tip atom 100 dump.ind_step2
run 6000

#equilibrate at 8000k
unfix 3
fix 5 tip temp/rescale 1 8000 8000 1 1

dump 3 all atom 100 dump.ind_step3
run 3000

#compute info for atoms in spherical tip

compute count_loc all coord/atom 1.77
compute count all reduce sum c_count_loc
#variable bond_count equal “c_count / 2”

#variable tot_energy equal thermo etotal
#variable pot_energy equal thermo pe

#fix outpress all print 100 “{ttime} {otemp} {count_limit} {bond_count} {tot_energy} {pot_energy}” file tip_info.txt

#quench to 300k
unfix 5
fix 7 tip temp/rescale 1 8000 300 .5 1

dump 4 all atom 100 dump.ind_step4
run 3000

#equilibrate at 300k

unfix 2
unfix 7

fix 8 all npt temp 300 300 100 iso 1000000 1 1000
dump 5 all atom 100 dump.ind_step5
run 6000

[sjplimp]

The 100 and 1000 in this command:

fix 8 all npt temp 300 300 100 iso 1000000 1 1000

are in units of picoseconds since you are using metal units.

That is far too long an equilibration time for a
simulation of 6000 timesteps = 6 psec.

Steve