Frequently asked exercise questions

Visualizing iso-surfaces

Iso-surfaces from data in a cube-file or a gpw-file can be visualized using Mayavi from the command line like shown here.

It’s a good idea to create a short alias like this:

$ alias iso="python3 -m ase.visualize.mlab -C gpaw"

so that you can simply do:

$ iso CO.cube  # plot cube file
$ iso slab-4.gpw  # plot electron density from gpw-file
$ iso slab-4.gpw -n 15  # plot wave function from gpw-file
$ iso -h  # help!

Making x-y plots

If you want to plot an x-y plot from data in a csv-file (comma separated values), you can use gnuplot:

$ gnuplot
gnuplot> plot "abc.csv" using 1:2

Alternatively, use this little Python script:

import sys
import numpy as np
import matplotlib.pyplot as plt
for filename in sys.argv[1:]:
    a = np.loadtxt(filename, delimiter=',').T
    x = a[0]
    for y in a[1:]:
        plt.plot(x, y, '-')
$ python3 <path-to-script>/ abc.csv

Writing 3-d data to cube files

This can be done from Python using the function:

from import write
write('abc.cube', atoms, data=data)

Square root

Square roots are calculated like this: 2**0.5 or sqrt(2) (the sqrt function must first be imported: from math import sqrt or from numpy import sqrt).

Integer division

In Python, / is used for both integer- and float divisions. Integer division is only performed if both sides of the operator are integers (you can always force an integer division by using //):

>>> 1 / 3
>>> 1 / 3.0

Why does changing one variable change another one?

The = operator in Python is not and assignment operator, it is a naming operator: It makes a new name for (reference to) the object:

a = [1, 2, 3, 4, 5]  # Create a list
b = a                # New name for list
a[2] = 42
print(b)             # [1, 2, 42, 4, 5]

c = 7
d = c
c += 42   # d is still 7, we just did
          # c = c + 42
          # creating a new object 49 and
          # giving it the name c

Saving plots

You can save plots made with matplotlib by pressing the floppy-disk icon in the bottom of the plot, and save as a .png file.

You can save a picture of the atoms from ASE’s GUI by choosing Save, and then specify a .png file.

You can view .png files in the databar with the command eog (“eye of Gnome”).