Cooperative stability, many-body expansion, and σ-aromaticity of (LiH) <i>n</i> clusters ( <i>n</i> = 1–6): A CCSD(T) study at the complete basis set limit

Abstract

We investigated the energetics and bonding of lithium hydride clusters (LiH)n (n = 1–6) using a composite ab initio scheme inspired by W2 theory to achieve sub-kcal/mol accuracy. This approach combines CCSD(T) results extrapolated to the complete basis set limit with a rigorous treatment of core–valence correlation, scalar relativistic effects, and the diagonal Born–Oppenheimer correction. Our results show that while Hartree–Fock theory captures the primary electrostatic binding, correlation effects are crucial for determining the energetic preference of compact isomers over cyclic rings. A parallel density functional theory study shows that while standard hybrid functionals like B3LYP-D4 and M06-2X exhibit larger deviations, the double-hybrid revDSD-PBEP86-D4 functional closely matches our benchmarks, delivering sub-kcal/mol accuracy. Structural and chemical bonding analyses, including intrinsic bond orbital, nucleus-independent chemical shift, and many-body expansion (MBE) methods, reveal high ionic character and multi-center bonding (3c–2e and 4c–2e) within the (LiH)n clusters. MBE analysis of the interaction energy of the monocyclic clusters with respect to the LiH molecules reveals that the two- and three-body terms are consistently negative (stabilizing), while all higher-order terms are negligible. We find that σ-aromaticity in these systems is predominantly local and bond-centered. As the rings expand, the interior becomes magnetically decoupled from the σ-skeleton, precluding the formation of a global ring current. These results establish definitive benchmarks for the stability of prototypical electron-deficient clusters.