Towards an understanding of the vibrational spectrum of the neutral Au7 cluster

We present a detailed theoretical study of the vibrational spectrum of the neutral Au7 cluster, aimed at understanding its reported experimental spectrum [P. Gruene et al., Science, 2008, 321, 674]. We study the effect of vibrational anharmonicity, polymorphism, noble gas embedding, and the use of various electronic-structure methods. We use a vibrational configuration-interaction approach (VCI) with a vibrational self-consistent field (VSCF) basis, in order to study the effect of vibrational anharmonicity for the density functional theory (DFT) global minimum energy structure. Our implementation of the VSCF/VCI method is based on the direct calculation of the potential energy surface (PES) using pseudo potential plane-wave DFT. An efficient reduction of the number of mode–mode couplings between vibrational modes (fast-VSCF/VCI) is used to speed up calculations. We show that the rather small anharmonicity does not account for the difference between harmonic and experimental frequencies and consequently for the large global scaling factor, reported by the authors of the experiment. Instead, the use of different electronic structure methods allows for a significant reduction of the scaling factor. We also show that krypton embedding does not significantly change the vibrational frequencies of the Au7 cluster.