We modeled SACs with various metal-coordinating atoms (i.e., N, O, P, and S) in vacancies formed in 5-and 6-membered rings (Figure ). The sizes of the 5-and 6-membered ring graphitic SAC systems investigated in this work are 59 and 61 atoms, respectively. Previous studies have shown that even though finite-size effects of SAC flake models are not negligible, the qualitative trends and comparison of dopant-dependent SAC properties will hold and DFT modeling of finite-size SACs permits the use of accurate (i.e., hybrid) DFT functionals that are cost-prohibitive when using periodic boundary conditions. To generate an expanded chemical space of SACs that are likely to be synthetically accessible by codoping the first and second coordination spheres, we define the local coordination environment of a SAC using three criteria: 1) the metal center; 2) the first coordination sphere atom identities (i.e., up to two dopant atom identities); 3) the second coordination sphere, which can contain dopants that are located in the smallest possible ring that contains all four metal-coordinating atoms (Figure ). We investigate two distinct codoped SACs configurations: codoping only in the first coordination sphere or also codoping the second coordination sphere (e.g., atoms directly connected to the first coordination sphere). For any given dopant configuration, we used up to two distinct atoms. First, we codoped the first coordination sphere without doping any atoms in the second coordination sphere. This results in four distinct positional dopant configurations for each of the 6 atom combinations (e.g., NO, NP, NS, OS, OP, PS) for each 5-and 6-membered ring SAC, resulting in 48 total arrangements (Supporting Information Figures and). The naming scheme for codoping in the first coordination sphere is ring size (e.g., 6-membered), metal and oxidation state (e.g., Fe(II)), and the identity and number of the dopants in the first coordination sphere (e.g., N2O2) along with any designation of symmetry (e.g., cis or trans) for the dopants where relevant (e.g., 6-membered Fe(II)(trans N2O2)). In the 6-membered ring SAC, the four dopants in the first coordination sphere of 6-membered rings is not symmetric (e.g., NOON and NNOO are inequivalent), but these modestly inequivalent variations were ignored due to their limited effect on energetics (Supporting Information Table ). For the second approach, we fixed the first coordination sphere to contain four N atoms, and codoped the second coordination sphere with two atoms that have the same identity (i.e., 2 N atoms, 2 O atoms, 2 P atoms, or 2 S atoms), following the doping reported in the experimental literature that tends to favor up to two unique elements and typically around two additional codopants in the second coordination sphere .