  The structure of oxide-supported metal nanoclusters plays an essential role
in their sharply enhanced catalytic activity over bulk metals. Simulations
provide the atomic-scale resolution needed to understand these systems.
However, the sensitive mix of metal-metal and metal-support interactions which
govern their structure puts stringent requirements on the method used,
requiring going beyond standard density functional theory (DFT). The method of
choice is coupled cluster theory [specifically CCSD(T)], but its computational
cost has so far prevented applications. In this work, we showcase two
approaches to make CCSD(T) accuracy readily achievable in oxide-supported
nanoclusters. First, we leverage the SKZCAM protocol to provide the first
benchmarks of oxide-supported nanoclusters, revealing that it is specifically
metal-metal interactions that are challenging to capture with DFT. Second, we
propose a CCSD(T) correction ($\Delta$CC) to the metal-metal interaction errors
in DFT, reaching comparable accuracy to the SKZCAM protocol at significantly
lower cost. This forges a path towards studying larger systems at reliable
accuracy, which we highlight by identifying a ground state structure in
agreement with experiments for Au$_{20}$ on MgO; a challenging system where DFT
models have yielded conflicting predictions.
