Amorphous SiO2 Unstable system

Hello everyone,

I’m trying to create amorphous silica(a-SiO2) by melting cristobalite SiO2 using tersoff potential. I followed the method mentioned in this [1] publication for the melting and quenching. The steps are briefly as follows, (Timestep is 0.5fs and pressure in all the npt ensembles were 1bar)

1: Create cristobalite at 1800K
2: Equilibrate the system for 0.5ns at 1800K under npt ensemble
3: Take the system into nve ensemble and check the stability
4: Heat the system to 5000K in npt with 1K/ps rate.
5: Equilibrate the system at 5000K in npt for 10ns.
6: Take the system into nve and check the stability.**
7: Quench the system to 300K at a rate of 0.1K/ps in npt.
8: Equilibrate the system for 1ns at 300K in npt.
9: Take the system into nve and check the stability.**

My problems are,

  1. At step NO 6, when I check the stability of the system at 5000K in nve, temperature of the system was increasing.
  2. At step NO 9, when I was checking the stability of the system at 300K in nve, temperature of the system was increasing.

I was able to solve both of the problems by reducing the timestep to 0.1fs. As my understanding, it should be possible to have energy conservation with a 0.5fs timestep in a-SiO2. But here I’am with a unstable system. Going back to 0.1fs timestep will cost me a lot of time in my calculations and so I really want to keep the stimestep at 0.5fs.

I would like to have some advice on the issue from someone who is familiar with a-SiO2. And is there any way of creating a-SiO2 by placing Si and O atoms randomly rather than heating and quenching method?
I have attached the input file of my simulation.
[1] -

I wish you well during this challenging time. Hope everyone keeps safe!

Thank you very much,

Chamara Somarathna. (2.92 KB)

Hi Chamara,

How did you come up with nine steps to make a-silica? The paper you referred didn’t mention the annealing procedure in details.

Did you visualize the model after step-5? Did you get reasonable density after step-5? I guess model becomes very diluted due to NPT at high temperature. Atoms move at high velocity, that’s why you see the increase in temperature in the subsequent NVE.

Following papers shows the detailed annealing procedure to make a-silica.

The first paper makes a-silica from silica crystal, while the 2nd one makes by randomly packing SiO2.