DFT and TD-DFT assessment of the structural and optoelectronic properties of an organic-Ag14 nanocluster

An extensive benchmark of exchange-correlation functionals on the structure of the X-ray resolved phosphine and thiolate-protected Ag14-based nanocluster, named XMC1, is reported. Calculations were performed both on simplified model systems, with the complexity of the ligands greatly reduced, and on the complete XMC1 particle. Most of the density functionals that yielded good relaxed structures on analogous calculations on gold nanoclusters (viz. those employing the generalized gradient approximation) significantly deform the structure of XMC1. On the contrary, some of the exchange-correlation functionals including part of the exact Hartree-Fock exchange (hybrid functionals) reproduce the experimental geometry with minimal errors. In particular, the widely adopted B3LYP yields fairly accurate structures for XMC1, whereas it is outperformed by many other functionals (both hybrids and generalized gradient corrected) in similar calculations on analogous gold-based systems. Time-dependent density functional calculations have been employed to recover the experimental UV-vis spectrum. The present investigation shows that to correctly reproduce the optical feature of XMC1 the ligands cannot be omitted, because they interact with the metal core at energies much closer to the optical gap than in the case of gold-based nanoclusters of similar sizes. Due to this fact, a functional that accurately describes charge-transfer electronic transitions (such as the long-range corrected CAM-B3LYP) has to be adopted.