Although the stabilization of the Au NPs in water is conditioned by the charged functional groups, the size of the obtained Au NPs depends mostly on the employed stabilizer, i.e., on the monomeric unit and the polymerization degree. Most notably, as compared to the triazine-based stabilizer (KCA), the stabilizers containing heptazine units (K,Na-PHI, KMel, KCya) lead to smaller Au NPs which also exhibit better stability (except for cyanurate) in high ionic strength electrolytes. We hypothesized that these beneficial effects of larger heptazine-based stabilizers might be due to the larger charge delocalization in the aromatic system, resulting in more effective electrostatic stabilization of Au NPs which are typically positively charged. To obtain further insight into the stabilization mechanism, we performed density functional theory (DFT) calculations to find the optimized geometry of KCA, KCya, and KMel structures and to investigate the impact of the conjugation size, negatively charged cyamelurate or cyanamide functional groups, and the nature and number of counterions (Fig. ). Particularly, we modeled the structures with a different number of counterions based on the assumption that the counterions initially stabilize the carbon nitride species and may detach after the stabilization of Au NPs. In addition, we performed a Fukui function analysis of the carbon nitride species to gain a deeper understanding of the stabilization mechanism of Au NPs. This analysis provides an estimate of the expected interaction between the stabilizer and Au-NP without the need to directly include the extended Au-NP (having a broad size and shape distribution) into the simulation. More specifically, the electron density of the 𝑓 -Fukui function of the carbon nitride represents the electron density for the electrophilic attack from the Au NPs (Fig. ). The optimized geometries of the carbon nitride species functionalized by the oxygen-containing cyamelurate groups (CA and Cya) reveal a structural out-of-plane deformation near the cyamelurate groups when the counterions are absent. However, in the presence of the counterions, the structures become more planar. In this context, it should be mentioned that the out-of-plane structural distortion translates into a breaking of the π-electron conjugation, which effectively reduces the size of electron delocalization. On the other hand, the cyanamide (NCN -)-functionalized species (melonate) tends to retain an in-plane structure. The presence of potassium counterions renders the structure even more planar, resulting in an extension of the electron conjugation as compared to the situation without the counterions. Therefore, the primary role of the counterions (potassium) can be understood as facilitating the flattening of the structures with functional groups. Specifically, the electron densities of 𝑓 -Fukui functions reveal that the presence of counterions causing the structural flattening leads to an alignment of the directional orbitals from the previously distorted structure into perpendicular π-orbitals within the flat structure. This orbital alignment translates into an extension of the delocalized electron size to the functional groups, facilitating thus the stabilization of the positive surface of Au NPs. The effectiveness of the extended π orbitals in stabilizing Au NPs is supported by the experimental observation that the stabilization (in terms of size and long-term stability) of Au NPs by the structurally more distorted KCA is similar to the case of stabilization solely by NaBH4, while the flatter carbon nitride species, such as KCya, K,Na-PHI, and KMel, can effectively stabilize Au NPs smaller than 3 nm in size (see Fig. and the histograms in ESI, Fig. ). In other words, these results suggest that the more effective stabilization of Au NPs by KCya, K,Na-PHI, and KMel as compared to KCA can be attributed to the availability of a delocalized electron size equivalent to or larger than the heptazine moiety. In addition, from the analysis of the condensed Fukui function (for details see Section 1.5. in ESI †), also known as the dual descriptor, we conclude that this phenomenon is a result of the synergistic effects of the functional groups and the counterions (Fig. ). The condensed Fukui function describes the local reactivity per atom, and its value is negative at sites prone to the electrophilic attack from an electrophile, and positive at sites preferable for the nucleophilic attack. Notably, the condensed Fukui functions for all atoms in structures with the same number of K + counterions as the number of functional groups yield all negative values, which corroborates their ability to effectively stabilize the electrophilic Au NPs (Fig. ). In contrast, the pristine (non-ionic, non-functionalized) heptazine and triazine moieties show positive values of the condensed Fukui functions despite their positive net charge (Fig. and S13 in ESI †), which highlights the significance of the presence of the counterion-balanced cyamelurate and cyanamide functional groups in ionic carbon nitride for the effective stabilization of Au NPs.