Cooperative many-body interactions and spectroscopic signatures in (HCN) _<i>n</i> and (HNC) _<i>n</i> clusters up to <i>n</i> = 15

We present a comprehensive computational investigation of hydrogen cyanide (HCN) and its higher-energy isomer, hydrogen isocyanide (HNC) clusters, from dimers to 15-mers. Clusters were generated using the PyAR automated build-up algorithm, which incrementally assembles n-mers from monomers <i>via</i> random orientations and Tabu-guided sampling, followed by BP86-D3/def2-SVP preliminary optimizations and final refinement at B97-D3BJ/def2-TZVPP. We applied energy decomposition to break down the binding energy into electrostatic, dispersion, and induction components. We used noncovalent-interaction maps to trace the formation of spiral and dual-ring hydrogen-bond networks. We examined bond-critical-point properties to track the incremental increase in bond strength that signals cooperative enhancement as cluster size grows. Our many-body decomposition goes further, isolating two-, three-, and four-body contributions to show that while pairwise interactions dominate, three- and four-body terms account for up to ∼20% of the total interaction energy-peaking at the trimer and then leveling off by <i>n</i> ≈ 6-and that HNC clusters consistently exhibit larger non-additive enhancements than HCN. Vibrational analysis further confirms this trend: as cluster size increases, the CH stretch in HCN and the NH stretch in HNC both shift steadily to lower wavenumbers, reflecting a gradual weakening of the isolated X-H bond and a corresponding strengthening of intermolecular hydrogen bonds. Additionally, QTAIM analyses confirmed an increase in electron densities and a positive Laplacian at bond critical points with increasing cluster size. Overall, these findings highlight the cooperative forces that drive unexpected isomeric stabilization and novel structural motifs-results directly relevant to astrochemical modeling of interstellar HCN/HNC abundance ratios and prebiotic chemistry in early Earth or cometary environments.