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#8 Easy to Get, but Contains a Lot of Information, Density of States(3)

Viewed : 686 times,  2019-07-09 11:13:17

The previous weekly tips #6 and #7 explained the definition and calculation method of DOS. This weekly tip looks at the importance of setting calculation parameters including the number of k-points in DOS calculations.

 

1. Correlation between input parameters and the DOS Graph

The following shows that the DOS graph of a silicon unit cell depends on the number of k-points; that is, the DOS graph is highly affected by the density of k-points.

 

Setting the density of the data point in the DOS graph is the'DeltaE' keyword in the DOS tab. The smaller this value is set, the closer the energy interval of DOS data is set.

However, when performing the DOS calculation, the larger the k-point grid is set, the more k-points are sampled. Therefore, the calculated more accurate eigenvalues ​​from the dense k-points make further smoother and broader graph shape.

 

There is another keyword to consider in addition to the k-point when setting the input script to get a DOS graph—“occupations.” This keyword decides how electron occupies the states.

“Smearing,” a default option in occupation, assumes that the model to be calculated is a conductor like metal. Gaussian smearing that places electrons slightly overflowing on the edges of the conduction band and valence band is used. “Fixed” assumes that the model is a nonconductor, so smearing is not considered.[1] “Tetrahedra” is an option for DOS calculations. It calculates an electron structure by dividing the reciprocal space into a tetrahedron in which a matrix element and the band energy is linearized. Because of its linear approximation, it is proper to analyze the complex structures of the Fermi surface.[2]

The following DOS graphs show the two cases—tetrahedra not in use and tetrahedra in use:

 

It can be found that setting tetrahedra as the occupation gets a slightly better result.

 

👉 Check the calculation results

 

 

2. Data Export

In the data page, the original raw data can be checked and downloaded. Searching “pdos” shows multiple files with the names in the type of output/VLAB.pdos_atom#1(Si)wfc#1(s).

 

DOS data is generated in the form of text, and each file is a DOS projected to all orbitals of all atoms of a calculation model. In this case, the number in front of an element symbol is set according to the order of the atoms and in the input script, and the number in front of the orbital is set according to the order of the orbital in pseudopotential regardless of the principal quantum number.

Click a file name to see the DOS data. In the data, “E (eV)” indicates an energy value that Fermi energy is not corrected. “ldos” indicates local dos and adds up pdos values. “pdos,” the DOS of each orbital, follows the procedure below:[3]

Adding up all LDOS values can get the total DOS. Find and open the VLAB.pdos_tot file to see the dos(E) row and the pdos(E) row. “dos(E)” is the value that means all LDOS values are added. “pdos(E)” means that all pdos values are added. This indicates that the two values are the same, and a slight difference between the two is caused by the rounding of a significant number.

The DOS data can be imported into other programs by copying and pasting it. Another way is to click the Download Button to download all original raw data of the calculation. Depending on the calculation, its file size may be very large. By opening the DOS data file in a different program, a DOS graph can be created.

For the past three weeks, we have looked at DOS from basic to in-depth information. As DOS contains a large amount of information even with easy calculations, it can be applied to diverse studies. Why don’t you make a DOS graph just by clicking several times with Materials Square?

 


[1] https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm321
[2] Blöchl, P. E., Jepsen, O., & Andersen, O. K. (1994). Improved tetrahedron method for Brillouin-zone integrations. Physical Review B, 49(23), 16223.
[3] https://www.quantum-espresso.org/Doc/INPUT_PROJWFC.html

 

 


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