I apologize for asking this stupid question, but I am a bit confused with the ISIF tag. Doesn't a cell's shape and cell's volume depend on the atoms' positions? If so, why can the cell's volume/shape and atomic position change independently in the ISIF tag?

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    $\begingroup$ 1/2 Doesn't a cell's shape and cell's volume depend on the atoms' positions? - Not really. The cell shape and size can be anything irrespective of atomic position. The cell is something that we repeat periodically in all the space dimensions. The atomic position doesn't have anything to do with it. I guess you are probably mistakenly assuming that the cell shape and size are determined by only the outer atoms! This is not the case. For example, you can have one C atom in a tetragonal cell at 0,0,0 with large lattice parameters a,b, and c. In this case, you are treating an isolated C atom. $\endgroup$ Apr 1 at 2:37
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    $\begingroup$ 2/2 But if you choose a and b to be large and c to be small, then you are treating an infinite C chain. Notice that you didn't change any atomic position. Still the C atom has 0,0,0 position. But since now you reduced the length of the cell in one direction (namely c - along z axis), the cell volume changed. Likewise you can change cell shape. Shape or volume do not depend on the atomic position. In ISIF tag, one controls how much degree of freedom one will allow in a relaxation calculations (for example one may want to let the C chain system to be relaxed only along z axis: ISIF=7) $\endgroup$ Apr 1 at 2:47
  • $\begingroup$ That explains my concern. Thanks a lot! $\endgroup$
    – 蕭力諶
    Apr 2 at 21:17
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    $\begingroup$ In that case, I will try to add it as an answer $\endgroup$ Apr 2 at 21:27


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