# Creating DLC via melting and cooling

Dear all,

I have written an input file which is to melt a diamond lattice at 8000K and cool down to 300K to create a diamond-like carbon.

However, I am having the problem that the atoms will melt and fly off. Also, I could not have set the pressure at 0 as the volume will blow up.

May I ask if there is any possible methods to improve on this problems?

Below are the input files which I have written.

atom_style full
boundary p p p
units metal
dimension 3
neighbor 2.5 bin
neigh_modify delay 1 every 1 check yes

atoms will melt and fly off

Since your system is periodic, what does this mean?
I suggest you first figure out how to do what you want
with NVT, using a reasonable fixed volume. Once you
do that and monitor what the pressure is at a couple
of tempteratures, you should be able to use NPT
and have it equilibrate nearly the same, assuming
you heat and expand it slow enough.

Steve

Hi.
Check ref [1] for a summary and a list of further references on ta-C
liquid quenching.
Instead of starting from a diamond lattice start from an unstable
lattice such as simple cubic. Also 8000K seems a bit high to me, when
starting from an unstable lattice equilibration of the liquid at 5000K
is sufficient (the temperature up-raming is not necessary either). If
you run at fixed volume then set the density of the system to match
the density of the ta-C (DLC) you want to achieve (that is obvious, I
guess...)
Best,
Daniel

[1] Marks, N., N. Cooper, D. McKenzie, D. McCulloch, P. Bath, and S.
Russo. “Comparison of Density-functional, Tight-binding, and Empirical
Methods for the Simulation of Amorphous Carbon.” Physical Review B 65,
no. 7 (January 2002).

Hi,

The coordinates of the atoms are all outside of the simulation box and it seems that the center of mass of the systems acquire a velocity causing them to cluster together outside the simulation box.

I tried doing it with NVT but it could not reach the temperature of 8000K that I want as the pressure will blow up instead and the simulation totally breaks down giving me a very large temperature then 0 all the way.

Thank you very much.

How can the coords be outside a periodic box?
If your atoms are moving too far in one or a few
timesteps (before you reneighbor) that they
can't be mapped back into the box, then your

Steve

Hi,

The problem with the crystal moving out has been solved. However, the volume of the crystal will still suddenly blow up to a large value causing the NVT fix to break down. Also, the pressure seems to fluctuate. Hence, I am having problem setting a pressure value for NPT fix.

atom_style full
boundary p p p
units metal
dimension 3
neighbor 2.5 bin
neigh_modify delay 1 every 1 check yes

I suggest you run with NVT and monitor the pressure
for a while. That will tell you what is a reasonable
value for P in an NPT simulation.

Steve

pair_style airebo 3.0 1 0
pair_coeff * * CH.airebo C

Dihedrals is an important part of AIREBO, why turning it off?

Best,
Ray