  A computational approach to determine the equilibrium structures of
nanoclusters in the whole temperature range from 0 K to melting is developed.
Our approach relies on Parallel Tempering Molecular Dynamics (PTMD) simulations
complemented by Harmonic Superposition Approximation (HSA) calculations and
global optimization searches, thus combining the accuracy of global
optimization and HSA in describing the low-energy part of configuration space,
together with the PTDM thorough sampling of high-energy configurations. This
combined methodology is shown to be instrumental towards revealing the
temperature-dependent structural motifs in Au nanoclusters of sizes 90, 147,
and 201 atoms. The reported phenomenology is particularly rich, displaying a
size- and temperature-dependent competition between the global energy minimum
and other structural motifs. In the case of Au$_{90}$ and Au$_{147}$, the
global minimum is also the dominant structure at finite temperatures. In
contrast, the Au$_{201}$ cluster undergoes a solid-solid transformation at low
temperature (< 200 K). Results indicate that PTMD and HSA very well agree at
intermediate temperatures, between 300 and 400 K. For higher temperatures, PTMD
gives an accurate description of equilibrium, while HSA fails in describing the
melting range. On the other hand, HSA is more efficient in catching
low-temperature structural transitions. Finally, we describe the elusive
structures close to the melting region which can present complex and defective
geometries, that are otherwise difficult to characterize through experimental
imaging.
