Melting Quartz to obtain amorphous silica

Dear Tom,

How are you constructing the table?

I am looking at teh Beest et al table right now, and I cannot reproduce the values in the tables . The first 4 values are listed for reference.

My values: (I get the same values from excel (plugging into BKS) or from pair_write in lammps). These are BKS for the Si-O
[index, time, potential, force]
1 0.01 -1.33538e+014 -8.01229e+016
2 0.316544 -129008 -2.4977e+006
3 0.447549 -14663.1 -213077
4 0.548088 -3742.4 -47994.5

Your table:
1 0.01 6.03e+049 1.44765e+053
2 0.316544 1.42e+014 1.22734e+016
3 0.447549 1.72968e+010 9.5173e+011
4 0.548088 1.13834e+008 5.00527e+009

The values are off by 30 orders of magnitude! There is no way any cutoff or turncation or even using a different force field would produce such a discrepancy.

I assume you used metal units, because that’s what the attached input script shows. I also assume you did not include coulomb in the table, becasue it is in the input file as well and mentioned somewhere in the thread.

With my current values, the simulation is exploding. I don’t understand why this is the case because the same values come from lammps and the excel spreadsheet, and I’ve used the metal units.

Also, what is your “24-6 LJ” that you added. I can’t find mention of it in Beest.

There is another thread on lammps that reports this exact same problem (they don’t explain it this way, they just say ‘lost atoms’) and it redirects here. I could, in principle, use the files written by you and those of Dr. Crozier but that would be a temporary, and in my opinion stupid, fix.

I sincerely appreciate any help.

Best,

hadi

Dear Paul,

Thank you very much, it seems like it is working with larger damping coefficients indeed.

I am just wondering. By doing this… Should the density of the final amorphous phase automatically adjust itself to that of amorphous silica?

I mean I am starting from a quartz phase with a density of about 2.6. Should that density naturally drop to about 2.2 for amorphous silica after the quench in NPH or is it not that straightforward at all?

When I quench it with your input script at 5000K for 3M steps of 0.9fs, I get a final phase of density 2.5… not quite 2.2 yet.

Should I rather start with a crystalline phase of density already modified to 2.2 to get closer to the real amorphous phase density?

what would be more realistic you think?

Thank you very much for all your insight on this.

Best regards,

Thomas Coquil
Research Assistant
UCLA Mechanical and Aerospace Engineering Department
420 Westwood Plaza
43-132 ENGINEERING IV
Los Angeles CA, 90095
Tel: 1 626 429 2110
[email protected]…36…24…

Hi Hadi,

What I did was to use hybrid overlay of lammps’ long coloumb buck and with a tabulated 24-6 Lennard Jones. You need to refer to later papers regarding this, it is to prevent atoms from getting too close at very high temperatures. You can look at Thomas et al 2011 for some info. Hope this helps.

Regards,
Yeo Jingjie
Ph.D. Student
School of Mechanical and Aerospace Engineering
Nanyang Technological University, Singapore