Unbiased structural search of small copper clusters within DFT

Highlights: • We have been able to identify novel metastable structures for small Cu clusters. • We have shown that a linear structure reported for Cu₃ is actually a local maximum. • Some of the structures reported in literature are actually unstable within DFT. • Some of the isomer structures found shows the limits of educated guesses. - Abstract: The atomic structure of small Cu clusters with 3–6 atoms has been investigated by density functional theory and random search algorithm. New metastable structures have been found that lie merely tens of meV/atom above the corresponding ground state, and could therefore be present at thermodynamic equilibrium at room temperature or slightly above. Moreover, we show that the previously proposed linear configuration for Cu₃ is in fact a local maximum of the energy. Finally, we argue that the random search algorithm also provides qualitative information about the attraction basin of each structure in the energy landscape.