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I am getting confused about what the electronic step and ionic step are in VASP. I used to think that finishing the following loop one time is called an electronic step. but I am not sure how to express the ionic step on the following diagram. enter image description here

According to some calculation experience and communication with others, now I tend to think that maybe each iteration during the iterative diagonalization procedure is called an electronic step, and finish "the solving Kohn-Sham equation Loop" is called ionic step, because when the program finishes this loop, it could adjust the atoms' position based on the Hellmann-Feynman theorem. enter image description here My question is: Which of these two thoughts is the right one?

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    $\begingroup$ Your first thought is correct, for each fix atom position we optimize charge density. If we are allowing atomic position relaxation, then for each atomic movement (ionic relaxation), we are doing electronic optimization. Changing atom position is ionic step $\endgroup$
    – Pranav kumar
    Dec 17, 2021 at 11:42
  • $\begingroup$ @pranavkumar, If so, every ionic step would only contain just one electronic step, but in the OUTCAR file that VASP generated, every ionic step contains dozens of electronic steps. Why is that? $\endgroup$
    – Jack
    Dec 17, 2021 at 11:54
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    $\begingroup$ @Jack what do you mean by one electronic step? When doing SCF calculations, there will be many steps till convergence for the ALL the electrons. $\endgroup$
    – Camps ♦
    Dec 17, 2021 at 12:58
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    $\begingroup$ For each ionic step, electronic step will keep on repeating until energy is converged. Once energy is converged ionic movement occurs and again electronic SCF loop continues. $\endgroup$
    – Pranav kumar
    Dec 17, 2021 at 13:12
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    $\begingroup$ @Camps, I see it now, that makes sense. $\endgroup$
    – Jack
    Dec 17, 2021 at 14:39

1 Answer 1

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Just to wrap up the discussion in the comments, your first image would depict an electronic step or SCF iteration where a set of orbital coefficients (equivalently, the electronic density) is calculated. These steps change how electron density is distributed within the molecule. The results of an electronic step are used as guess for the next electronic step (assuming the density hasn't converged).

An ionic step would occur after a full cycle of SCF iterations has completed. This involves moving the nuclei/ions of the system based on the forces calculated from the converged electronic wavefunction/density. Once the nuclei/ions have been moved, you have to start a new electronic cycle to figure out where the electrons should go for the updated geometry.

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