If I tell you that NaCl has the rocksalt crystal structure, can you (or DFT) tell me what is the lattice parameter?
Your task is now to apply this to the Fe-Al crystal for which you got a starting cif file on the previous page. Prepare a series of 5 input files for your DFT code, with the unit cell rescaled (*) such that the volumes of these 5 cells are 20%, 15%, 10%, 5% and 0% smaller than the initial volume. Calculate the total energy of the unit cell for these 5 cases (medium precision is fine, expect 2-3 minutes per volume on a regular laptop), and make a plot of the total energy as a function of unit cell volume. How would you determine from this plot the preferred volume for this Fe-Al crystal? Make a final cif file with that particular volume. Write all of these down in the document you will upload at the end of the chapter.
(*) By rescaling we mean that the b/a and c/a ratios between the lattice parameters are kept identical to the values in the initial cif. Also the unit cell angles and the atom positions are kept identical to the initial values.
If you know about scripting, you can make a script to run these 5 calculations in one go. But it’s perfectly fine as well to do this as 5 separate cases.
You might find inspiration in the examples on geometry optimization in part C of the course (although up to here you don’t strictly need those yet).
Please indicate in the form hereafter whether or not you could make this task successfully.
If you ran into a difficulty that you cannot solve, please report it in the forum hereunder. If you ran into a difficulty that you could solve, please share the solution with us too — it may help others. Feel free to post any other comment or thought about this exercise. And if you can answer questions posted by fellow students, please help them out.