Creating DLC via melting and cooling

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1

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

2

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

3

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).
http://link.aps.org/doi/10.1103/PhysRevB.65.075411.

4

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.

Please advice on this matter.

Thank you very much.

5

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
dynamics are very bad.

Steve

6

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

7

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

8

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