Two-to-three dimensional transition in neutral gold clusters: The crucial role of van der Waals interactions and temperature

Abstract

We predict the structures of neutral gas phase gold clusters (Aun,n=5-13) at finite temperatures based on free-energy calculations obtained by replica-exchange ab initio molecular dynamics. The structures of neutral Au5-Au13 clusters are assigned at 100 K based on a comparison of experimental far-infrared multiple photon dissociation spectra performed on Kr-tagged gold clusters with theoretical anharmonic IR spectra and free-energy calculations. The critical gold cluster size at which the most stable isomer changes from planar to nonplanar is Au11 (capped-trigonal prism, D3h) at 100 K. However, at 300 K (i.e., room temperature), planar and nonplanar isomers may coexist even for Au8, Au9, and Au10 clusters. Density-functional theory exchange-correlation functionals within the generalized gradient or hybrid approximation must be corrected for long-range van der Waals interactions to accurately predict relative gold cluster isomer stabilities. Our paper gives insight into the stable structures of gas phase gold clusters by highlighting the impact of temperature, and therefore the importance of free energy over total-energy studies, and long-range van der Waals interactions on gold cluster stability.