Total energy calculation with different lattice symmetries

[opvu]

Dear vasp users,

I want to calculate groundstate structure of tetragonal-BaTiO3 and rhombohedral-BaTiO3

But I don't know how to set the unit cell in tetragonal phase or rhombohedral phase.

Whether it is right or not that:
We set the lattice symmetry in the POSCAR via atomic positions?

thanks very much!

<span class='smallblacktext'>[ Edited ]</span>

[Veronika]

First read the manual. You define your cell via the POSCAR file
http://cms.mpi.univie.ac.at/vasp/vasp/node63.html

If you still need to know how to set it up you can have a look here:
http://cst-www.nrl.navy.mil/lattice/