Finding a Needle in a Haystack: Smart Calculation of Characteristic Vibrations in Large Molecules

Abstract

Vibrational spectroscopy is a powerful tool to investigate the struc- ture and dynamics of molecular systems. When large molecules are stud- ied, quantum chemical calculations help to interpret the spectra. In many cases, experimental questions are related to specific regions of a vibrational spectrum, so that an assignment is only required for a subset of vibra- tions. 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 deter- mined. However, the selective calculation of the relevant information only can be made much more efficient by using mode-selective techniques such as 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. This guess defines the scientific problem which is to be studied. As examples, we review the calculation of adsorbate modes of a flexible thiophenolate anion attached to a rigid silver cluster model- ing an extended silver surface. It reveals that mode-tracking works very well for such systems and results in a considerable gain of computational efficiency. The second example is the mode-selective calculation of Ra- man Optical Activity intensity differences for the breathing modes of two different diastereomers of a right-handed deca-alanine helix.