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

苏锐 su_rui at outlook.com
Wed May 22 15:44:46 CEST 2013


Hello Jan,


Thanks for you advices.

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



发件人: Jan M. Knaup
发送时间: ‎2013‎年‎5‎月‎22‎日, ‎星期三 ‎21‎:‎28
收件人: User list for DFTB+ related questions


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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