Role of Anharmonicity and Bimetallic Intermixing in the IR and Raman Spectra of Cun₍ and Pdn₍Cu6−n₆-₍ Clusters (n<7n 7). Ab Initio Benchmarking of DFT

Accurate prediction of vibrational spectra of subnanometric transition‐metal clusters is essential for their experimental identification and for understanding structure–property relationships relevant to catalysis and nanomaterials design. Here, we present a high‐level ab initio benchmarking study of harmonic vibrational wavenumbers for copper and mixed copper–palladium clusters containing up to six atoms, providing reliable theoretical references for infrared (IR) and Raman spectroscopic characterization. Reference data obtained from CCSD(T) with complete basis set extrapolation, explicitly correlated CCSD(T)‐F12, and multireference internally contracted Rayleigh–Schrödinger second‐order perturbation theory (RS2C) calculations are used to assess the accuracy of widely adopted density functional theory (DFT) approaches applicable to larger clusters and compositionally complex systems. Analysis of bare copper and mixed PdCu () clusters reveals a composition‐driven evolution of chemical bonding, from molecule‐like to increasingly multicenter, delocalized, metal‐like behavior, with direct consequences for vibrational fingerprints. Moreover, we provide high‐level ab initio evidence, for the first time, that Pd in its lowest‐energy triplet state adopts a Jahn–Teller–distorted octahedral geometry. Using the resulting well‐benchmarked DFT framework, we map the evolution of IR and Raman spectra as a function of cluster size and composition and rationalize these trends in terms of underlying electronic structure and symmetry of the clusters. Significant anharmonic effects on vibrational wavenumbers and intensities further underscore the need for accurate theoretical frameworks to support the spectroscopic assignment and functional understanding of subnanometric transition‐metal clusters in realistic environments.