Convergence Tools: mesh cutoff

This type of project diagnoses the eggbox effect to be expected for a particular pseudopotential/basis-set combination. The energy as a function of atomic coordinates displacement is computed. The eggbox period is evaluated for a set of selected Mesh cutoff values.

The project starts with the definition of the atomic structure. We refer the user to section Structure Modeling in ASAP for further information on ASAP structure builder/viewer interface.

After the atomic structure is defined, edit the type of project.

From the list of possible project types implemented in ASAP, select Convergence tools: mesh cutoff.

Then click on the Parameters icon to open the Convergence tools: mesh cutoff parameters widget.

The project parameters that can be tuned are:

• Mesh cutoff values in eV units. The Mesh cutoff parameter defines the plane wave cutoff for the grid. Press the Add value button to incorporate to the right window, each of your selected Mesh cutoff values. In this type of project, ASAP will run a set of single-point calculation using the selected Mesh cutoff values.

  

  

• Directions. You can select the direction in which you desire to evaluate the eggbox effect; 1-st, 2-nd and 3-rd unit cell vector. You must be aware that the number of calculations, and the computational time, will be doubled (tripled) when two (three) directions are selected.

  

• Number of steps. In this type of project, the energy is computed as a function of atomic coordinates displacement. This field indicates the number of rigid displacements.

  

  

Click on the Calculator icon to select the computational engine to be used during geometry optimisation.

The Convergence Tool: mesh cutoff is only compatible with SIESTA calculator. We refer the user to chapter The SIESTA Calculator for further information on SIESTA calculator. Notice that, the real-space mesh cutoff energy located in the Convergence parameters widget, described in subsection SIESTA Parameters: Convergence is disabled for this type of project.

Click on the Run icon to open Run widget. Then click on the Run button to submit the set of calculations required to evaluate the eggbox effect for the selected meshcutoff parameters.

We refer the user to chapter Runners for further information on computational resources configuration in ASAP.

After submitting a job (run), the tab Calculator output in Run widget shows the complete calculation output in real time.

In addition, the tab Task output in Run widget shows relevant information of Geometry optimisation output in real-time.

Analysis

When the calculation is completed, select Exit and analyse to open the analysis widget.

Select the Convergence of the mesh cutoff… button to visualise the energy as a function of atomic coordinates displacement.

Check the tick-boxes corresponding to the mesh cutoff values and directions for which you desire to evaluate the eggbox effect. The maximum energy difference (\(\Delta E_{max})\) is indicated in the figure for each of the selected mesh cutoff values.

You can also visualise the effective force as a function of atomic coordinates displacement.