QuantumATK Forum

QuantumATK => General Questions and Answers => Topic started by: tfpper on October 4, 2012, 01:57

Title: Geometrical optimisation of Si nanosheet
Post by: tfpper on October 4, 2012, 01:57
What is the best way to optimise the geometry of a nanosheet of Si atom.
From the tutorial, I've got the following.
Should I use the default ATK-DFT or ATK-Extended Hückel?

Code
calculator = LCAOCalculator()

bulk_configuration.setCalculator(calculator)
nlprint(bulk_configuration)
bulk_configuration.update()
nlsave('C:/Users/davng/ATK/Silicene/SiliceneSheet10_0_10_Buckle2_OptGeom.nc', bulk_configuration)


bulk_configuration = OptimizeGeometry(
        bulk_configuration,
        max_forces=0.05*eV/Ang,
        max_steps=200,
        max_step_length=0.5*Ang,
        trajectory_filename='C:/Users/davng/ATK/Silicene/SiliceneSheet10_0_10_Buckle2_OptGeom_traj.nc',
        disable_stress=True,
        optimizer_method=QuasiNewton(),
        )
nlsave('C:/Users/davng/ATK/Silicene/SiliceneSheet10_0_10_Buckle2_OptGeom.nc', bulk_configuration)
nlprint(bulk_configuration)
Title: Re: Geometrical optimisation of Si nanosheet
Post by: zh on October 4, 2012, 06:59
To get a reliable results before production run for a specific system, the users must do some test calculations for the setup of k-point mesh and the mesh cutoff by themselves. Usually, the ATK-DFT is more reliable than the ATK-Extended Huckel.
Title: Re: Geometrical optimisation of Si nanosheet
Post by: tfpper on October 4, 2012, 07:19
OK. I got this from another ATK tutorial. Do you mean to add this code into the script. What is the default k_point_sampling?
Also, as a test calculation, what is a good sample size in the case of a nanosheet?

Code
numerical_accuracy_parameters = NumericalAccuracyParameters(
    k_point_sampling=(9, 9, 1),
    )

calculator = LCAOCalculator(
    numerical_accuracy_parameters=numerical_accuracy_parameters,
    )
Title: Re: Geometrical optimisation of Si nanosheet
Post by: zh on October 4, 2012, 07:46
Yes, you should add such setup into your script.  The default k_point sampling is 1 x 1 x 1, see
http://quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/ref.numericalaccuracyparameters.html
.
It is difficult to say the  good sample size for a general nanosheet, because the unit cell size and the electronic properties (metallic or semiconductor) of the nanosheet may vary case by case.
Title: Re: Geometrical optimisation of Si nanosheet
Post by: Anders Blom on October 4, 2012, 09:33
Huckel cannot be used for optimizations since it has no repulsive part of the pair potential. You could use DFTB if you obtain access to the relevant parameters, it's at least a lot faster than DFT, and reasonably accurate.