(253k) Role of Van Der Waals and Entropy on Gold Cluster (meta)Stability

Discerning the ground state structure of gas phase gold clusters, and in particular the critical size when gold clusters begin favoring three-dimensional (3D) structures over two-dimensional (2D) structures at 0 K, has been a topic of longstanding interest. At finite temperature, however, gold clusters exhibit many structural isomers of similar energies and can interconvert between them. To move beyond the monostructure description of clusters at 0 K, we performed ab initio replica-exchange molecular dynamics (REMD) on Au_n clusters (n =5-14) to identify metastable states, their relative populations between 100-400 K, and to examine the influence of entropy and van der Waals (vdW) on the isomer energetic ordering. REMD calculations used PBE with the scalar ZORA relativistic correction and Many-Body-Dispersion (MBD) vdW. Free energies of Au_n isomers are optimally estimated using the Multistate Bennett Acceptance Ratio, and the 2D and 3D isomer populations are examined at different temperatures and cluster sizes. The distribution of bond coordination numbers and radius of gyration are used to address the challenge of discriminating isomers along the dynamical trajectories. Dispersion effects are shown to be critical to obtain the correct isomer energetic ordering, stabilizing 3D structures relative to 2D structures by up to 0.4 eV. Entropic effects typically stabilize metastable 3D structures relative to planar/quasiplanar structures. Computed IR spectra of Au_n are compared with experimental spectra obtained via far-IR multiple photon dissociation in a molecular beam at 100 K, which enables the structures of Au₉, Au₁₀, and Au₁₂ to be assigned.