Modelling the structural evolution, electronic structures and optical properties of Sc₆Se_n^0/- (n = 1-12) clusters

The size-dependent structural evolution of scandium-selenium cluster remains poorly understood, particularly regarding the origin of growth transitions and the emergence of stable building blocks. Here, neutral and anionic Sc₆Se_n^0/- (n = 1-12) clusters were systematically investigated by global search techniques combined with B3LYP density functional theory calculations, with additional single-point validation at the B3LYP-D3(BJ) and DLPNO-CCSD(T) levels. The results reveal a clear two-stage growth behavior with n = 8 as the critical turning point. For n = 1-8, Se atoms sequentially occupy the eight electrostatic potential minima sites on the Sc₆ octahedral core, ultimately forming the highly symmetric Chevrel-phase Sc₆Se₈ cluster. This transition point is governed by both geometric and electronic factors. As a result, the clusters undergo structural reorganization for n = 9-12, giving rise to competing link-like and pentagonal-pyramidal motifs. Stability analyses based on average binding energy and second-order energy difference identify Sc₆Se₈ as the most distinguished cluster in the series. Simulated photoelectron, vibrational, and ultraviolet-visible spectra provide characteristic fingerprints for structural identification. In particular, Sc₆Se₈ exhibits pronounced aromaticity, strong visible-light absorption, and favorable excitonic features, highlighting its role as a magic super-atomic building block in the Sc-Se system.