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When calculating the band structure of a material, I have been told that at first, I need to do structure optimization to get a stationary structure(1st step), and then keep the atoms' position fixed, do a self-consistent calculation to get the band structure(2nd step). But if I am doing this job with VASP, self-consistent field calculation of the electrons is included in the loop when optimizing the structure. The only difference is at the first step of optimizing structures, the atoms' position is variant, but when getting to convergence, I am guessing the atoms' positions don't vary too much actually
My question is:
Is there a difference between the charge density of the 1st step and 2nd step?

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The procedure that takes more time to finish is the geometry optimization (in each step there are self-consistence field calculations).

There are properties that can not be calculated at the same time (all in one step) or you need to know more information about how to calculate it. For example, to calculate the DOS, you need to know the band structure to then, select the energy interval.

Answering your question: Is there a significant difference between the charge density of the 1st step and 2nd step? No.

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  • $\begingroup$ So after structure optimization, I don’t need to keep the atoms’ position fixed and do self-consistent calculation again. I could just do non self consistent calculation to get band structure? $\endgroup$
    – Jack
    Dec 15, 2021 at 18:29
  • $\begingroup$ No, for each property, you need to do self-consistent field (SCF) calculation. But if your code save the SCF data, you can just load it and use it. Also, you have to remember to replace the atom's positions in your input file with the optimized ones. $\endgroup$
    – Camps ♦
    Dec 15, 2021 at 18:53
  • $\begingroup$ But the standard procedure is 1-structure optimization,2-SCF calculation,3-non-SCF calculation to get band structure. I am actually wondering if I could just do 1 and 3. I just don’t understand why all the lecture and tutorials recommend three steps. $\endgroup$
    – Jack
    Dec 15, 2021 at 19:00
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    $\begingroup$ Often you want a dense k-point grid in the SCF (step 2) prior to step 3 to get a smooth band structure (or what ever quantity you are computing). But performing the structure optimization with a dense grid is expensive. So separating step 1 and 2 is mainly for computational reasons. $\endgroup$
    – Fabian
    Dec 17, 2021 at 0:28
  • $\begingroup$ @Fabian, I checked the vasp wiki. it says: Since the charge density written to the file CHGCAR is not the self-consistent charge density for the positions on the CONTCAR file, do not perform a bandstructure calculation (ICHARG=11) directly after a dynamic simulation (IBRION=0). Here is hyperlink:vasp.at/wiki/index.php/CHGCAR $\endgroup$
    – Jack
    Jan 7, 2022 at 16:11

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