where θ is the angle between the H-H bond and the z-axis connecting the center of the molecule This suggests that the number of unsaturated Cu I sites present locally per cluster affects the dynamics of that cluster upon H2 adsorption, which in turn effects the enthalpy of adsorption, as observed in DFT calculation for cluster models with different number of Cu I sites. This may also partly contribute to the broadened peaks observed in the infrared spectra.