what does relaxing mean？

[dyc_2008]

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？

[alex]

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

[vasp_user]

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?

[dyc_2008]

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