Atomistic origin of defect states. Experimentally, it is difficult for scientists to definitively observe the presence of surface defects due to their dilute concentration, which is presently beyond the resolution of most characterization techniques. Therefore, theoretical insights are indispensable to unveil the atomistic identity underlying the obtained spectroscopic fingerprints. The first error to consider is the proper representative model for describing the defect in a host material. The embedding-potentials provide quantitative accuracy but is technically challenging to implement due to the finite cluster approach in many-body methods. The supercell approach is the most widely employed method in first principle calculations, including density functional theory (DFT), to describe an isolated defect. Therefore, the computational challenge of treating a relatively small region of correlated defect states within the expanded supercell while maintaining the periodic boundary conditions is thus ideal to separate defects from periodic interactions between adjacent images. For example, if we were to expand a cubic primitive cell with lattice vector (a=b=c) into a supercell based on a transformation scalar matrix with determinant k 3 , the resulting repeat unit will have a spacing of k × a between each defect along a given axis. It is then straightforward to introduce either vacancies or substitutions simply by removing atoms and replacing atoms, respectively. However, interstitial sites require further attention, and one should not rely solely on observing the host crystal structures to determine them. For instance, interstitial sites can be assigned such that it either emulates the well-known structure motifs of tetrahedra and octahedra or are based on a Voronoi tessellation scheme . Alternatively, we can assign the interstitial sites based on their atomic boundaries through the volumetric data of the host material (i.e., considering the all electron charge density). In addition, one should also take into account the possible charged states of defect sites, e.g., Zn vacancies in ZnO have been demonstrated to exhibit the +1 charge state owing to the capturing of multiple polarons at the neighboring O sites ; The density of Se vacancies in Sb2Se3 with +2 charge state increases under Se-rich synthesis condition owing to the defect-correlation compensation mechanism .