Step 2 - Set up a Configuration

To create our silica configuration we’ll use the “relative random mix” generator as a basis.

Configuration ⇨ Create… ⇨ Relative random mix

Choose both the silicon and the oxygen species and continue

Double-click the new configuration tab and rename it to Bulk

Let’s set-up the parameters in the generator first of all. The relative random mix uses the population of the first species (referred to as A) as the reference, and sets the population of all other species relative to the first, based on user defined ratios. So, here we will use the silicon species as A and set its population explicitly, and set the relative population of species B (the oxygen) to be twice that of A, thus giving us SiO₂.

Open the settings for the Parameters node

Change the value of the integer populationA variable to 1500

Set ratioB to 2

The density of bulk, amorphous silica is 0.0664 atoms Å^-3. We use units of atoms Å^-3 here as this allows us to add both atomic components using the same density value. If we had chosen units of g cm³ we would have to know the individual densities of silicon and oxygen in amorphous silica.

Change the value of the real rho variable to 0.0664.

Next, the box. We will still use relative lengths and scale the box according to our species population and density, but we will create a monoclinic box rather than a cubic one:

Open the settings for the Box node

Set the angle γ to 120 degrees

Finally, we just need to check the ordering of the species in the Add nodes.

Open the settings for the first Add node

Make sure that the target Species is Si

Open the settings for the second Add node

Make sure that the target Species is O

We’re all set up, so click Regenerate to recreate the box using our new parameters.

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