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I think the position of each atom in any crystal is fixed（of course，no crystal is perfect）。
Do we relax atoms into perfect position？
But what does this mean？

Hi, have you thought about taking a lesson in this topic? That surely helps understanding many things!

Maybe this will help. I'm going to assume that you are neglecting anything like vibrations of the atoms in the crystal.

So you are pointing out the following: if I can look up the lattice parameters of a well known crystal, then why do I have to do a relaxation in VASP? Furthermore, if VASP relaxes the electrons, finds that there is a force on the atoms, and then relaxes the atoms into positions other than the "perfect" positions that I input into the run at the beginning, then doesn't that mean that VASP screwed up a perfect structure? Here is my answer: Not necessarily.

Let me explain. What we are trying to discover by going through a procedure like this is, given the approximations any large scale quantum calculation must use, and given the input parameters I have chosen, what is the theoretical value I get for the crystal. This output crystal structure I get can then be compared to the "perfect" or "well known" value I used to start the calculation.... to see how close my theory value is to the value I found in the literature. The value in the literature could be wrong by the way and your VASP number could be better... or vice versa, of course. Does that help?

dear vasp_user :
thank you so much . this dose help me so much.