Direct targeting of adsorbate vibrations with mode-tracking

The normal modes and vibrational frequencies of a thiophenolate anion adsorbed on an Ag(1 1 1) surface are directly targeted employing the mode-tracking protocol. The adsorbate normal modes of three model systems are compared. These three models consist of an Ag51 cluster with a thiophenolate in a fixed orientation attached to it and are geometry-optimized either completely or only with respect to the adsorbate structure or not at all. Vibrational normal modes and frequencies calculated with mode-tracking are found to be equal to the results of a full frequency analysis. The calculation of vibrational modes and frequencies in harmonic approximation is shown to be justified for partially optimized structures as long as only the harmonic frequencies of those modes are interpreted which solely involve movements of the atoms belonging to the optimized subsystem. Adsorption of the thiophenolate results in adsorbate frequency shifts which are reproduced equally well by any of the three geometrically different models.