Hello dear researchers, I hope you are doing well.
I want to ask you a question. I have a unit cell with 24 atoms (4 A, 4 B and 16 X) and I want to substitute X atom by another atom (for example changing one X by other element).
My question is, can I substitute it within the unit cell without making supercell?? or should I make a supercell??
Hamza Elaamri
Hello! When substituting an atom within a unit cell in quantum espresso (QE), whether you need to create a supercell depends on various factors, including the size of the substituted atom compared to the original X atom and the specific properties you're investigating.
In general, if the substituted atom significantly alters the unit cell's dimensions or introduces strain, creating a supercell may be necessary to maintain the system's integrity. Additionally, if the substitution leads to a change in the system's symmetry, a supercell may be required to accommodate these changes.
However, if the substituted atom is of a similar size and chemical nature as the original X atom, and the substitution does not significantly affect the unit cell's properties, you may be able to perform the substitution within the existing unit cell without creating a supercell.
It's essential to consider the specific requirements of your simulation and the desired accuracy of your results when deciding whether to use a supercell or perform the substitution within the unit cell. Experimentation and careful analysis of the system's behavior can help guide your decision-making process.
Ultimately, the best approach may vary depending on the specific characteristics of your system and the goals of your research. Feel free to provide more details if you need further assistance!
Best regards,
Sandeep
Sandeep Jaiswal thank you so much for your response
I will substitute O by S, I think it will change properties, but for symmetry I don't have an idea. How can I know from QE that the symmetry is changed or not?
Hamza Elaamri
You can search for 'Sym. Ops.' keyword in the output of pw.x to check if the symmetry operations are changed by the substitution.
Yue-Wen Fang thank you
I found this
4 Sym. Ops., with inversion, found
What does it mean??
The need of a supercell arises from two considerations:
1) In a small cell, the substitution of a single atom might strongly change the atomic configuration, forcing possible reconstructions. With a supercell, this effect is diluted.
2) You might wish to get closer to the experimental evidence, where the level of doping is relatively small. You need to estimate if your doping is adding free electrons to the unit cell and how many such electrons you might have. It is unfortunately difficult to move away from the degenerate limit when you have too many free electrons. In Si, for example, the degenerate limit is about 10^20 e/cm^3.
However, a large supercell requires heavy calculations...
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