In the structure selection step, we select the silicon structure from the examples.

Tasks

Click on the "From examples" tab
Select Bulk silicon from the dropdown list
Click the Confirm button to proceed

Warning: If the confirmed structure is not yet stored in the AiiDA database, it will be stored automatically when you proceed to the next step.

Warning: Changes after confirmation will reset the following steps.

In this second step (Configure workflow), we define the parameters needed to perform our phonon dispersion calculation.

Tasks

Select Full geometry, i.e. we optimize both cell parameters and atomic positions. This is important when computing phonons, as we are interested in the curvature of the energy at its minimum (i.e., where forces are zero).
Open Step 2.1 for further instructions

Here we can customize the calculation parameters. The settings are divided into several tabs. Two tabs are always present:

Basic settings: top-level calculation settings
Advanced settings: fine-tune the calculations

Additionally, each plugin activated in the previous step 2.1 can add additional tabs. Note that the app pre-configures defaults for most parameters.

See further instructions in the Basic settings panel below.

Here we define the basic settings of the simulations.

Tasks

Since we are simulating silicon, select the Insulator electronic type
Select the Fast protocol
See further instructions in the Vibrational settings panel

Note: For the purpose of completing this guide in short time, we will use the Fast protocol for default parameters. However, you if dispose of enough computational resources, you can also select Balanced or Stringent.

In the Vibrational settings panel, we can customize the parameters for the phonons calculation.

Tasks

Select Phonons for non-polar materials and INS in the dropdown menu. (you can learn more about the other options, including IR and Raman spectroscopy, in the corresponding in-app guide.)
Set the Supercell size to be [2,2,2], if not already set.
Estimate the number of supercells to see the number of supercells needed to compute finite displacement forces. The output number should be equal to 1, thanks to symmetry between different displacements.
Click on the Confirm button to proceed

Note: a [1,1,1] supercell computes phonons only at the Γ point. This is appopriate, e.g., for computing IR and Raman spectra. However, it is not sufficient for interpolation in the whole Brillouin zone to correctly reproduce phonons dispersion: at least a [2,2,2] supercell is usually needed, and often a larger one for converged results..

Warning: Changes after confirmation will reset the following steps.

In the submission step, we define the computational resources to be used in the calculations. The global resources are used to define resources across all workflow calculations. Optionally, you can override the resource settings for specific calculations.

Warning: If running locally (for example, on the AiiDAlab demo server), we recommend keeping nodes and CPUs at the default minimum of 1 each.

Once the resources are defined, we can optionally customize the workflow label (pre-filled according to the settings of steps 1 & 2), as well as provide a detailed description of the workflow. Once we are ready, we can submit the workflow. You first need to select which code (code executable + computer where this will run) to use for each step of the workflow. The Quantum ESPRESSO app should always install a local Quantum ESPRESSO executable that is sufficient for this tutorial, but you can setup additional codes installed on remote supercomputers. For more information on how to set up codes, please refer to the corresponding documentation.

Tasks

Select 1 node and 1 CPU for each of the codes below
(Optional) customize the workflow label
(Optional) add a workflow description
Click the Submit button to proceed

Warning: The workflow may take a moment to submit.

Below you can monitor the status of the calculation, that might take several minutes to conclude even if we are using the fast protocol. Click on the Results panel. In the results panel, each tab will open results pertaining to a specific calculation submitted by the workflow.

See further instructions below in the Vibroscopy results tab.

When phonons results are available, the Load results button will become active, allowing us to load the results from AiiDA.

Tasks

Click on the Phonons tab to visualize the phonon spectrum and thermodynamic properties.
After having inspected the Phonons tab, click on the Neutron scattering tab to start and inelastic neutron scattering analysis.

Here we can inspect the phonons spectrum and the thermodynamic properties computed within the workflow.

Tasks

Check the phonon band structure and verify that there is a degenerate mode around 12.11 THz for q=X.
Feel free to zoom in and out and note how the phonon DOS is synchronized with the phonon band structure panel. Note that to zoom out, you can hover on the plot and use the little "Home" icon that appears on the top right of the plots (more tools are also available there).
Verify that the computed specific heat (constant volume) at 300 K is more or less 39.73 J/(mol K).
Go to the Neutron scattering tab.

Here we obtain the dynamical structure factor spectrum S(Q, ω) for inelastic neutron scattering (INS).

Tasks

Click on the Initialise INS data to compute the INS signal (it will just take a few seconds).
Change units to be THz.
Decrease the q step to be 0.01 Å^-1 and increase the number of energy bins #E bins to be 1000. Then, click on the Replot button to update the plot.
In order to see all phonon modes regardless of the value of S(Q, ω), select the Plot mode to be DOS map, replot and verify that you can find the same degenerate mode seen in the phonons spectrum.
Play with the other parameters to change the plot as you prefer. In particular, you can read more instructions clicking on the Plot info button. Try to use the custom k-points path specified in those instructions.

Post-guide exercises

In the Neutron scattering results tab, switch to Powder sample and try to perform similar analysis as done for a single crystal.
- Verify that a signal is still present at the same frequency as the degenerate mode detected before.
- Enhance the low intensity signal by using the Intensity window (%) slider.
Use the app to compute the phonon band structure of SiO₂ (silicon oxide). Compare the results with the ones obtained for silicon. In the Vibrational settings, select the same simulation type as in this example.