# [DFTB-Plus-User] Help on melting quartz to obtain amorphous silica

Thu May 23 03:40:51 CEST 2013

```Dear Jan,

Many thanks。 I will try your suggestions.

Regards

Su

Dear Su,

I try to steer clear of NPT ensembles. You would probably need to significantly increase the k-point sampling. Even then, pressures are known to oscillate wildly. Since for generating an amorphous model, the pressure is not very important, I would advise you not to bother with a barostat.

It's more important to ensure that you really achieve melting and that you thermalize your melt long enough to be uncorrelated with the initial crystalline structure.

Incidentally, maybe it would be better for you to start with a random distribution of atoms rather than a crystal structure. I understand you are interested in getting an amorphous model, not the dynamics of melting or amorphization. Starting from a stable crystal structure might not be helpful at all.

Best,

Jan

Jan M. Knaup                      | Fon +49-(0)421-218-62351
Dipl. Phys. Dr. rer. nat.         | Fax +49-(0)421-218-62770
Universität Bremen - BCCMS        |
Am Fallturm 1                     | Jan.Knaup at bccms.uni-bremen.de
28359 Bremen                      | JanKnaup at gmail.com
Germany                           | www.bccms.uni-bremen.de

2013/5/22 苏锐 <su_rui at outlook.com>

Hello Jan,

Just as you pointed out, I start from the beta cristobalite phase which owns a similar density (2.2 g/cm3) as the glassy state. In fact, i also gave a try for the npt essemble at first and found the pressure is very hard to be equilibrated. The pressure oscillated from about -10 GPa to 10GPa. Is there some advice on that problem?

Key part of the npt input:

Driver = VelocityVerlet {
MovedAtoms    = 1:-1

TimeStep [fs] = 0.5

Thermostat = NoseHoover {
Temperature [Kelvin] = TemperatureProfile {

constant    1    300
constant    1000 300
linear      2000 4000
constant    2000 4000
exponential 5000 300
}
CouplingStrength [cm^-1] = 3000
}
Barostat = {
Pressure  [Pa] = 1.0e5
Timescale [fs] = 10
}
OutputPrefix = "amorphous_md"
}
The number of timesteps is set very small for test purpose.

Regards

Su

Dear Su,

you have keep in mind two important factors:

First, the melting temperature depends strongly on the pressure, which is connected to the supercell volume. Since people mostly simulate at optimized or experimental 0K cell volumes, the melting point is shifted upwards considerably. 4000K is not necessarily high enough for an oxide material at 0K volume. The 1800K melting temperature you mention is the isobaric melting temperature at 1 atmosphere pressure. I'm sure you will find a much higher pressure in your simulation box.

Second, in most cases, amorphous phases of a material have a lower density than the crystalline ones. Depending on the specific behavior of SiO2, at the relatively high equilibrium density of your starting material, the crystalline phase may just be very favorable over the amorphous.

From both these consideration folows, that you might want to reconsider your simulation cell volume and rescale to either the density of your amorhpous target phase or the density at the melting temperature.

Additionally, you don't have to think your material is melted, you can know quite precisely. To decide, you can analyze the radial pair distribution function, the self diffusion or the Lindemann index of your simulated cell.

Best,

Jan

Jan M. Knaup                      | Fon +49-(0)421-218-62351
Dipl. Phys. Dr. rer. nat.         | Fax +49-(0)421-218-62770
Universität Bremen - BCCMS        |
Am Fallturm 1                     | Jan.Knaup at bccms.uni-bremen.de
28359 Bremen                      | JanKnaup at gmail.com
Germany                           | www.bccms.uni-bremen.de

2013/5/22 苏锐 <su_rui at outlook.com>

Hello,

I have looked the structure evolution, I think the system got melted at 4000K.  But the structure seems to be re-crystalized during cooling procedure. Considering that the melting point of quartz is about 1800K, I think 4000K should be sufficiently large to push the system out of FE minima. So maybe 4000K is too low for generating bulk defects?

Thanks

Su

Hello,

melting tends to proceed from defects, so its quite possible that your annealing never left the free energy
minima of the perfect crystal (just ending up super-heating the crystobalite structure above its melting
point and then cooling down again). Did you look at the structure at high temperatures?

Regards

Ben

On 22/05/13 02:51, 苏锐 wrote:

Hi!

I am now trying to obtain amorphous silica through the melting and quenching procedure. The initial system is built using a 2x2x2 supercell of beta-cristobalit containing 192 atoms. I meet a problem that the quenched structure is “defect free”. That means no non-bridging oxygen atoms or 5-member rings etc. that might exist in amrphous structure. I think there might be some error in my md input. Would someone have a look at my input and give some advices?

Here is my input:

Geometry = GenFormat {
<<< silica222.gen
}

Driver = VelocityVerlet {
MovedAtoms    = 1:-1
TimeStep [fs] = 1.0
Thermostat = NoseHoover {
Temperature [Kelvin] = TemperatureProfile {
# initial temperature = 4000K
constant    1    4000
# melt at 4000K for 5ps = 5000 step
constant    5000 4000
# reduce to 3000 in 20ps
exponential 20000 300
# equilibrium at 300K
constant    5000 300
}
CouplingStrength [cm^-1] = 3000
}
MDRestartFrequency = 100
OutputPrefix       = "amorphous_md"
}

Hamiltonian = DFTB {
SCC = Yes
SlaterKosterFiles = {
Si-O  = "./skf/Si-O.skf"
O-Si  = "./skf/O-Si.skf"
Si-Si = "./skf/Si-Si.skf"
O-O   = "./skf/O-O.skf"
}
MaxAngularMomentum = {
O  = "p"
Si = "d"
}
Filling = Fermi {
Temperature [Kelvin] = 300.0
}
KPointsAndWeights = {
0.0 0.0 0.0 1.0
}
}

Options {}

ParserOptions {
ParserVersion = 4
}

--
Dr. B. Hourahine, SUPA, Department of Physics,
University of Strathclyde, John Anderson Building,
107 Rottenrow, Glasgow G4 0NG, UK.
+44 141 548 2325, benjamin.hourahine at strath.ac.uk

Strathclyde 2012 THE Awards UK University of the Year

The University of Strathclyde is a charitable body,
registered in Scotland, number SC015263

_______________________________________________
DFTB-Plus-User mailing list
DFTB-Plus-User at dftb-plus.info
http://www.dftb-plus.info/mailman/listinfo/dftb-plus-user

_______________________________________________
DFTB-Plus-User mailing list
DFTB-Plus-User at dftb-plus.info
http://www.dftb-plus.info/mailman/listinfo/dftb-plus-user
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.dftb-plus.info/pipermail/dftb-plus-user/attachments/20130523/6aae1ba2/attachment-0001.html>
-------------- next part --------------
_______________________________________________
DFTB-Plus-User mailing list
DFTB-Plus-User at dftb-plus.info
http://www.dftb-plus.info/mailman/listinfo/dftb-plus-user
```