Finding a needle in a haystack: direct determination of vibrational signatures in complex systems

Vibrational spectroscopy is a powerful tool to investigate the structure and dynamics of molecular systems. When large molecules are studied, quantum chemical calculations are used to interpret the spectra. In many cases, experimentally driven questions are related to specific regions of a vibrational spectrum, so that an assignment is only required for a subset of vibrations. This holds true in particular for biomolecules, inorganic compounds which are stabilized by many bulky ligands, or other extended systems. In standard quantum chemical calculations of the vibrational spectrum, all normal modes and frequencies of the molecule under study are determined. However, the selective calculation of only relevant information can be made much more efficient by using mode-selective techniques as provided by the mode-tracking algorithm. A critical point for the performance of the mode-tracking scheme is the preparation of a guess vibration, which is then iteratively refined. The guess defines the scientific problem which is to be studied. Various examples are presented to highlight this aspect and the features of the mode-tracking algorithm in general.