Calculations were performed using the Vienna ab initio Simulation Package (VASP) version 5.4.4. For all calculations, only considered the Γ point was considered when sampling the first Brillouin zone. Periodic spin-polarized DFT+U calculations were performed with a 400 eV cutoff energy, a plane-wave basis, and the PBE functional coupled with a semi-empirical D3 dispersion correction with Becke-Johnson damping on a CHA supercell containing 12 T-sites. Electron-ion interactions were described with projector augmented wave (PAW) method. For the +U calculations, the U value for the d-orbitals of Cu was set to 6.0 eV based on a recent benchmarking study by Chen et al. Lowest energy Cu dimer structures were sampled over 150 ps 300 K ab initio molecular dynamics (AIMD) simulations with NVT ensemble with Nosé-Hoover thermostat. Unique local minima for each Cu dimer motif were identified by optimizing 400 structures (equally spaced in time). The structures were relaxed until all forces were smaller than 0.01 eV/Å, with a convergence criterion of 10 -8 eV for energies inside self-consistent-field (SCF) cycles. Raman intensity calculations followed the same protocol described by Liang et al. For Cu dimer structures with multiple unique local minima, spectra were Boltzmann-averaged to compute the final spectra reported in Figures and: