Neutral gold clusters studied by the isothermal Brownian-type molecular dynamics and metadynamics molecular dynamics simulations

The DFTB theory was combined with the isothermal Brownian-type molecular dynamics (MD) and metadynamics molecular dynamics (MMD) algorithms to perform simulation studies for Au clusters. Two representative DFTB parametrizations were investigated. In one parametrization, the DFTB-A, the Slater–Koster parameters in the DFTB energy function were determined focusing on the ionic repulsive energy part, E_rep and the other, the DFTB-B, due attention was paid to the electronic band-structure energy part, E_band. Minimized structures of these two parametrizations were separately applied in MD and MMD simulations to generate unbiased and biased trajectories in collective variable (CV) space, respectively. Here, we found the MD simulations monitored at 300 K manifest fluxional characteristics in planar cluster Au₉/DFTB-A, but give no discernible tracts of fluxionality for planar Au₈/DFTB-A and Au₈/DFTB-B, for nonplanar Au₁₀/DFTB-A and, to some extent, for nonplanar Au₉/DFTB-B; they are plausibly being hindered by higher-than k_BT energy barriers. Very recent FIR-MPD spectroscopy measurements, however, were reported to have detected at 300 K both the planar and nonplanar neutral Au_n clusters in the size range 5 ≤ n ≤ 13. The failure of MD simulations has prompted us to apply the MMD simulation and construct the free energy landscape (FEL) in CV space. Through scrutinizing the FELs of these clusters and their associated structures, we examine the relative importance of E_rep/DFTB-A and E_band/DFTB-B in unraveling the covalent-like behavior of valence electrons in Au_n. Most important of all, we shall evaluate the DFTB parametrization in MMD strategy through comparing extensively the simulation data recorded with the gas-phase experimental data.