# Lattice vectors are misaligned and vacuum size is reduced, was my calculation successful

I did an optimization for a monolayer with GGA (550 eV ecut, 12 12 1 k-mesh) with ISIF=3 and IBRION=2.

My original POSCAR was:

5.2916193008         0.0000000000         0.0000000000
0.0000000000         5.2916193008         0.0000000000
0.0000000000         0.0000000000        20.0097122192


After optimization, my CONTACR is this:

 5.2753749526661906    0.0000000006352637    0.0000000700426255
-0.0000000006164817    5.2753749536648096   -0.0000000640637957
0.0000002625200859   -0.0000002421078825   19.9912853387061134


You can see my lattice vectors (off-diagonal) has some non-zero values now and vacuum size is reduced a bit.

Do you think my CONTCAR is okay for further calculations, or should I worry about that?

NOTE: I did not fix any axis for the optimization.

• For a 2D system, you can't use ISIF=3.
– Jack
Aug 22 at 13:55

You used ISIF = 3, which tells VASP you want to relax the atomic positions and the cell shape/volume. It looks like the cell shape/volume has changed slightly during the relaxation process, as you noted. Based on this information, the input lattice constants yield a structure with higher energy than those in the converged CONTCAR. It seems like VASP is doing precisely what you asked it to do.
On a related note, if you have a monolayer with lots of vacuum in the $$z$$-dimension, it does not seem wise to relax all three lattice constants. You probably want to keep the vacuum dimension fixed.