Core electrons, unlike valence electrons, are hardly involved in a chemical interaction and are not significantly changed during the interaction. Thus, if the state of a core electron is considered as chemically inert, then we can approximate the potential by expressing only for valence electrons. By this, you can efficiently use computing resources with minimal effect on the accuracy of the calculation. The potential created in this process is called pseudopotential. 
For efficient DFT calculation, we introduce pseudopotential. Pseudopotential contains various information, necessary for simulating a system. Thus, you need to select a proper pseudopotential for each atom.
1. SSSP Pseudopotential
In Quantum Espresso (QE), you can select various pseudopotentials. In main libraries, you can get pseudopotential as follows:
- Standard Solid State PPs (SSSP)
- Optimized Norm-Conserving Vanderbilt Pseudopotential (ONCVPSP)
Currently, the pseudopotential file, provided by Materials Square by default, is the pseudopotential of the SSSP library.
The SSSP library suggests a general standard protocol that verifies a pseudopotential library publicly available. This is done by comparing the calculation results obtained by using diverse pseudopotentials with multiple verification items such as cohesive energy, pressure, and band structure. The SSSP library with the pseudopotentials selected by applying these standards is a good open-source pseudopotential library, suitable for high-throughput material screening and high precision material modeling.
Moreover, the SSSP library provides two types of pseudopotential sets: efficiency and precision. They have different verification standards according to the purpose of using each set. In case you use the precision set, its verification standards are higher than those of the efficiency set.
As the precision set is designed to assume that the accuracy would be high, it is suitable for a calculation with high accuracy. However, the same pseudopotential may exist in both the efficiency and precision sets. In general, as the file name of a pseudopotential follows a specific rule, if there is the same file name in the pseudopotential file list, it will be the same pseudopotential file. In this case, as they are the same file, select any set file from efficiency or precision.
Currently, SSSP ver 1.1 (efficiency) is set by default in Materials Square. Select the “Potential” tab in the Quantum Espresso module, and click the “Magnifying Glass button.”
Then, the list of available pseudopotential files will be displayed.
In MatSQ 3.0, you can directly upload a pseudopotential file. For more information on this function, see Potential Management
In case a customized pseudopotential is added, a prefix “custom_,” is also displayed in front of a library name. Select it to carry out a calculation by applying the pseudopotential added to the atom selected from the QE module.
2. Comparison of DFT Results Obtained by Using Different Pseudopotentials
When comparing DFT calculation results, you should not directly compare the energy values of the calculation results using different pseudopotentials. Despite the pseudopotential of the same atom, as each pseudopotential has a different option or condition, using a different type of pseudopotential results in different energy value. However, as its tendency is maintained, it is recommended to compare tendencies by finding the difference of values when comparing DFT results.
The chart below is the cohesive energy calculation result, obtained by using different pseudopotentials for a silicon bulk unit cell. The two calculations use the same structures and input scripts, but each uses Ultrasoft (SSSP 1.1 efficiency) and PAW type pseudopotential (PSlibrary and Si.pbe-n-kjpaw_psl.1.0.0.UPF). The calculation result shows that the energy obtained from the structure has a big difference. However, in calculating cohesive energy, the difference is offset, and the results of the two pseudopotentials become similar.
Some pseudopotentials are suitable for a specific system such as a fully relativistic potential, suitable to consider the spin-orbit effect or handle heavy atoms, as well as a potential that allows a non-collinear calculation. With these pseudopotentials, calculations are made under a specific condition; you can get a band structure or DOS, or different from that when using a different pseudopotential.
This weekly tip features the pseudopotentials provided by Materials Square. There are many types of pseudopotentials, and they affect calculation results. Thus, it is recommended to consider a proper pseudopotential to obtain desired data.
In particular, you can easily add a desired pseudopotential and carry out a DFT calculation as much as you want with MatSQ 3.0.
Apply the pseudopotential suitable for the structure you’ve researched with MatSQ 3.0!
 Martin, R. M., & Martin, R. M. (2004). Electronic structure: basic theory and practical methods. Cambridge university press.
 Prandini, G., Marrazzo, A., Castelli, I. E., Mounet, N., & Marzari, N. (2018). A Standard Solid State Pseudopotentials (SSSP) library optimized for precision and efficiency (Version 1.1, data download). Materials Cloud Archive.
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