question on berry phase calculation

[yingma]

Dear all,

I'm using VASP to calculate the self polarization of BaTiO3. I checked the OUTCAR file and I found that the strucutrue in OUTCAR is different from that reading in POSCAR. Specifically,
my POSCAR file contains a tetragonal cell with c/a=1.01, however, the OUTCAR give a structure of a orthorhombic cell with b/a=1.0001 and c/1=1.01. And the lattice vectors changed from (a, 0, 0) to (0, -a, 0), etc.
So my question, why the structure changed in a berry phase calculation? What's the physics behind this? Thanks A LOT!
<span class='smallblacktext'>[ Edited ]</span>

[Shahla Chowdhury]

I think u put ISIF=3 which relaxes both ion and volume.
But I also have the same problem of lattice vector change (a, 0, 0) to (0, -a, 0) .... I need a BCT structure with two metal.

I put
a
1 0 0
0 1.4 0
0 0 1.4
Direct
0 0 0
0.5 0.5 0.5
in my POSCAR file. But I think I need to change the POSCAR file because I am not getting the lattice parameter a. May I know what should the INCAR, POSCAR and KPOINTS files be!