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how to set the POSCAR file？
I mean，how we konw the coordinate of each atom？
should we test the cell to get coordinates by ourselves？

If possible, use available experimental data as a starting point for the coordinates. If you are not familiar with any experimental studies beforehand, you can e.g. have a look in Landolt-Börnstein for references. Once you have a starting point, you can then coninue to relax the cell parameters and internal parameters with appropriate IBRION, POTIM and ISIF options.

If you just know the lattice structure, and no experimental volume are available (e.g if you want to look at metastable structures or alternative magnetic orderning), you will have to screen different cell volumes until you find the equilibrium. To look up other theoretcial results if available, can also give you a hint on where to start.

Cheers,
/Dan

dear forsdan :

You got a thanks from china ：“谢谢â€�