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

Jan M. Knaup Jan.Knaup at bccms.uni-bremen.de
Wed May 22 15:27:25 CEST 2013


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
>
> *发件人:* Ben Hourahine
> *发送时间:* ‎2013‎年‎5‎月‎22‎日, ‎星期三 ‎15‎:‎43
> *收件人:* User list for DFTB+ related questions
>
> 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
>
>
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