Intensity tracking for theoretical infrared spectroscopy of large molecules.

We present an approach for the direct calculation of vibrational normal modes with high infrared intensities based on a mode-tracking-like algorithm [M. Reiher and J. Neugebauer, J. Chem. Phys. 118, 1634 (2003)] but with distinct features: no collective guess vibration is utilized but high-intensity distortions are constructed. Only the modes of interest with the highest infrared intensities are then targeted irrespective of a predefinition of the underlying collective normal coordinates. This leads to a fast access to the most important features in infrared spectra. The different implementations of the mode selection procedure are validated on a set of small organic molecules as well as on the metal complex Delta(deltadeltadelta)-tris(ethylenediaminato)cobalt(III) and the peptide all-(S)-decaalanine. As a critical test case, approximate infrared spectra of Schrock's dinitrogen molybdenum complex are calculated via intensity tracking.