CO on Small Metal Clusters: Site-Resolved Vibrational Shifts and DMRG and VQE Benchmarks of Electron Correlation

Mohsen Doust Mohammadi

Published Oct 21, 2026• 0 citations

Abstract

Strong electron correlation in transition-metal chemistry motivates both advanced classical solvers, such as the density matrix renormalization group (DMRG), and quantum algorithms, but a claim of quantum utility requires a converged classical baseline. This baseline is established for a concrete problem, the C–O stretching signature of CO on small Mn clusters (M = Cu, Ag, Au, Pd, Pt; n = 2–5). Shifts and adsorption energies of 35 complexes from PBE0/def2-TZVPP full numerical Hessians, corrected with CCSD(T) and tested against six further functionals, are organized by site coordination: top-site CO shifts by +6 to −108 cm −1 , bridgesite CO by −202 to −387 cm −1 , and hollow-site CO by −339 to −508 cm −1 , separated by at least 94 cm −1 , more than the estimated uncertainty of 15–20 cm −1 . Multireference diagnostics test the classical hierarchy. CASSCF with 8 and 12 active orbitals gives linear minima for CuCO and AuCO, which contradicts the bent minima of PBE0. DMRG for eight cluster complexes with up to 36 active orbitals and 61 electrons changes by 0.5 m E h or less between bond dimensions 500 and 1000 and takes 0.3–2.8 h per complex. Seven complexes are weakly entangled (largest single-orbital entropy 0.21–0.26), whereas top-site Pt5CO is moderately multiconfigurational (0.62). A noiseless UCCSD variational quantum eigensolver (VQE) reproduces CASSCF in 16-qubit active spaces only to within 2.0–4.3 m E h , above chemical accuracy. For these systems classical methods suffice, and entanglement screening with DMRG is proposed as the test that a candidate system must pass before quantum advantage is sought.

DOI: 10.26434/chemrxiv.15010810/v1

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