MatSQ Blog > Quantum-espresso

#7 Easy to Get, but Contains a Lot of Information, Density of States(2)

2019-06-18 11:49:34

The last weekly tip described the definition of DOS and its application cases. This time, we will look at how to get DOS data using Materials Square.

 

1. How to Get DOS Data using MatSQ

Quantum Espresso provides “projwfc.x,” a postprocessing code for calculation of DOS. Projwfc.x is used to calculate the projection of the wave function of an atomic orbital.[1] The DOS graph is obtained by plotting the projected DOS data with the energy (E – EF) that Fermi energy was corrected. In MatSQ, DOS is calculated by adding a DOS tab in the Quantum espresso module, it makes users easily get DOS data together with the SCF calculation (PWSCF).

A template, which is a workflow set prepared for users who are not familiar with calculation, can be added at the top of the Work page. Adding a DOS template to see its data displays the calculation step guide on the upper section, and by looking at the guide, you can check which step is performed now.

Draw a structure to get DOS in Structure Builder, and press the next module. Then, the Quantum Espresso module for density functional theory (DFT) calculation is activated. As DOS calculation needs a denser k-point grid than that of general energy/structure calculations, it is found that the selection of DOS sets larger k-points than energy/structure calculations. For information on the k-point setting, see the last weekly tips, #2 and #4.

 

2. Data Interpretation by Adding DOS Module

After checking the green, end message in the Quantum Espresso module, click the next module to activate the DOS module. Then, check the DOS data.

The DOS module is composed of the structure visualizer and the atom/orbital list. The calculation of DOS gives the projected density of states (PDOS), a DOS projected to each atom of each orbital with the total DOS. At that time, select a desired atom/orbital from the list and add it to see its PDOS. Selecting some atoms from the visualizer displays only the orbital of the atoms in the list. Mouse dragging simply changes the x-axis range in the chart, and by right-clicking, it opens the chart setting menu to modify the graph color and legend.

The graph shows that DOS depends on energy. At the time, a large DOS indicates that many electrons can occupy the energy level. On the other hand, if DOS is 0, there are no states that electrons can occupy in the energy level.

In the case of insulators, there is bandgap near the Fermi level. In the bandgap, which is an energy gap between the conduction band and the valence band, DOS is 0. In other words, no states that electrons can occupy exist. Thus, it is found that energy, as much as the minimum bandgap, is needed to transfer electrons in the valence band to the conduction band.[2],[3]

 

3. DOS Calculation Considering Spin Polarization

In the QE module, set the nspin keyword to 2, and perform a calculation considering the spin polarization to get a DOS graph with both spin up and spin down.

 

An electron has a magnetic moment by spin angular momentum and generates a magnetic property. As the last weekly tip mentions, the DOS with spin up/down is distinguished by using a symbol. As a substance without magnetic properties has the same number of states where spin up/down can occupy, DOS is symmetrical in the DOS=0 straight line. However, in case of an asymmetric DOS, the number of states where spin up/down can occupy is not the same, which means the substance has magnetic properties.[3]

We have learned how to calculate DOS using Materials Square, so the next article will cover the difference in graph shapes depending on DOS calculation parameters, and how to export and interpret original raw data files.

 


[1] https://www.quantum-espresso.org/Doc/pp_user_guide.pdf
[2] Suib, S. L. (Ed.). (2013). New and future developments in catalysis: Solar Photocatalysis. Newnes.; https://en.wikipedia.org/wiki/Density_of_states
[3] Spaldin, N. A. (2010). Magnetic materials: fundamentals and applications. Cambridge University Press.

 

 


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