Bulk aluminum

We will look at bulk fcc aluminum and make a single energy calculation at the experimental lattice constant \(a_0\) = 4.05 Å. For the first example, we choose a plane-wave cutoff energy of 300 eV and 4 x 4 x 4 k-points. Copy this Al_fcc.py to a place in your file area:

"""Bulk Al(fcc) test"""
from __future__ import print_function
from ase import Atoms
from ase.visualize import view
from gpaw import GPAW, PW

name = 'Al-fcc'
a = 4.05  # fcc lattice parameter
b = a / 2

bulk = Atoms('Al',
             cell=[[0, b, b],
                   [b, 0, b],
                   [b, b, 0]],
             pbc=True)

view(bulk)

k = 4
calc = GPAW(mode=PW(300),       # cutoff
            kpts=(k, k, k),     # k-points
            txt=name + '.txt')  # output file

bulk.set_calculator(calc)

energy = bulk.get_potential_energy()
calc.write(name + '.gpw')
print('Energy:', energy, 'eV')

Read the script and try to get an idea of what it will do. Run the script by typing:

$ python3 Al_fcc.py

The program will pop up a window showing the bulk structure. Verify that the structure indeed is fcc. Try to identify the closepacked (111) planes. In ase gui this is done by choosing View ‣ Repeat.

Notice that the program has generated two output files:

Al-fcc.gpw
Al-fcc.txt
  • A tar-file (conventional suffix .gpw) containing binary data such as eigenvalues, electron density and wave functions (see Restart files).
  • An ASCII formatted log file (conventional suffix .txt) that monitors the progress of the calculation.

Try to take a look at the file Al-fcc.txt. Find the number of plane-waves used. You can conveniently monitor some variables by using the grep utility. By typing:

$ grep iter Al-fcc.txt

you see the progress of the iteration cycles including convergence of wave functions, density and total energy. If the txt keyword is omitted the log output will be printed directly in the terminal.

The binary file contains all information about the calculation. Try typing the following from the Python interpreter:

>>> from gpaw import GPAW
>>> calc = GPAW('Al-fcc.gpw', txt=None)
>>> bulk = calc.get_atoms()
>>> print(bulk.get_potential_energy())
-4.12234332252
>>> density = calc.get_pseudo_density()
>>> density.shape
(9, 9, 9)
>>> density.max()
0.42718359271458561
>>> from mayavi import mlab
>>> mlab.contour3d(density)
<mayavi.modules.iso_surface.IsoSurface object at 0x7f1194491110>
>>> mlab.show()

Equilibrium lattice properties

We now proceed to calculate some equilibrium lattice properties of bulk Aluminum.

  • First map out the cohesive curve \(E(a)\) for Al(fcc), i.e. the total energy as function of lattice constant \(a\), around the experimental equilibrium value of \(a_0\) = 4.05 Å. Get four or more energy points, so that you can make a fit.

    Hint

    Modify Al_fcc.py by adding a for-loop like this:

    for a in [3.9, 4.0, 4.1, 4.2]:
        name = 'bulk-fcc-%.1f' % a
    

    and then indent the rest of the code by four spaces (Python uses indentation to group statements together - thus the for-loop will end at the first unindented line). Remove the view(bulk) line.

  • Fit the data you have obtained to get \(a_0\) and the energy curve minimum \(E_0=E(a_0)\). From your fit, calculate the bulk modulus

    \[B = V\frac{d^2 E}{dV^2} = \frac{M}{9a_0}\frac{d^2 E}{da^2},\]

    where M is the number of atoms per cubic unit cell: \(V=a^3/M\) (\(M=4\) for fcc). Make the fit using your favorite math package (NumPy) or use ase gui like this:

    $ ase gui bulk-*.txt
    

    Then choose Tools ‣ Bulk Modulus.

    Another alternative is to use the Equation of state module (see this tutorial).

  • Compare your results to the experimental values \(a_0\) = 4.05 Å and \(B\) = 76 GPa. Mind the units when you calculate the bulk modulus (read about ASE-units here). What are the possible error sources?

    Note

    The LDA reference values are: \(a_0\) = 3.98 Å and \(B\) = 84.0 GPa - see S. Kurth et al., Int. J. Quant. Chem. 75 889-909 (1999).

Convergence in number of k-points

Now we will investigate the necessary k-point sampling for bulk fcc Aluminum; this is a standard first step in all DFT calculations.

  • Repeat the calculation above for the equilibrium lattice constant for more dense Brillouin zone samplings (try k=6,8,10,...).

    Hint

    You may want to speed up these calculations by running them in parallel.

  • Estimate the necessary number of k-points for achieving an accurate value for the lattice constant.

  • Do you expect that this k-point test is universal for all other Aluminum structures than fcc? What about other chemical elements ?

Equilibrium lattice properties for bcc

  • Set up a similar calculation for bcc, in the minimal unit cell.
  • Make a qualified starting guess on \(a_\text{bcc}\) from the lattice constant for fcc, that you have determined above. One can either assume that the primitive unit cell volumes of the fcc and bcc structure are the same or that the nearest neighbor distances are the same. Find a guess for \(a_\text{bcc}\) for both assumptions. Later, you can comment on which assumption gives the guess closer to the right lattice constant.
  • Check that your structure is right by repeating the unit cell.
  • Map out the cohesive curve \(E(a)\) for Al(bcc) and determine \(a_\text{bcc}\), using a few points. Is it a good idea to use the same k-point setup parameters as for the fcc calculations? Calculate the bulk modulus, as it was done for fcc, and compare the result to the fcc bulk modulus. What would you expect?
  • Using the lattice constants determined above for fcc and bcc, calculate the fcc/bcc total energies. The total energies that GPAW calculates are relative to isolated atoms (more details here: Total Energies). This exercise is sensitive to the number of k-points, make sure that your k-point sampling is dense enough. Also make sure your energies are converged with respect to the plane-wave cutoff energy.