I have a little problem concerning hexagonal volume fitting using VASP. I have followed these steps:
Relax the structure from a given volume using
Copy CONTCAR to POSCAR and relax it again
ISMEAR = -5and without relaxation
Repeat for different volumes (+10% +5% 0% -5% -10%)
Use your script for EOS fitting.
It worked successfully with cubic systems. But for a hexagonal system (e.g. Carbon), I got the figure below. I don't know why I always get wrong results for the first volume point. Even with 7 volume points the first point always wrong.