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

[苏锐]

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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