First of all, I assume that you are taking VASP to perform your calculation.
Secondly, I assume that your structural defect is taking a Hf atom from your structure. (You can deal with substitutional doping with similar logic.)
Thirdly, for the HfS2 monolayer, there are two phases, namely T-phase and H-phase. The T-phase monolayer was fabricated in the experiment, however, the phonon spectrum indicates that the H-phase monolayer is thermally unstable. So I assume that you are considering the defects problem in T-phase HfS2 monolayer.
What is important for this type of calculation?
- When the T-phase is doping, you should take the spin-orbit coupling into account due to the broken inversion symmetry and the existence of heavy atom Hf.
- To simulate HfS2 monolayer, a large vacuum (20 angstroms) should be included along the z-direction.
- You should relax your doped structure to find the lowest-energy configuration.
- Defects may induce magnetism in your system, you should make a spin-polarized calculation to verify that.
How large a supercell guarantees that I have no interactions between individual defects?
A $4\times 4\times1$ supercell is enough. You can ref this paper, in which the author investigated the monolayer T-phase PtSe2 with substitutional doping.
What traps can I fall into?
- Without building supercell.
- Without consideration of spin-orbit coupling.
- Without adding enough vacuum along the z-direction.
- The lattice constant is important, you should take experimental lattice constant to build your model not taking bulk lattice constant.