Chromophore-specific theoretical spectroscopy: From subsystem density functional theory to mode-specific vibrational spectroscopy

Spectroscopy forms the bridge between theory and experiment in the analysis of structure, properties, and reactivity of functional molecules and molecular aggregates. Our knowledge on the basic working principles of systems such as photosynthetic units strongly relies on spectroscopic information, which is interpreted in terms of molecular or submolecular building blocks. To choose such entities as the essential ingredients in a quantum chemical framework is thus a promising route to the theoretical spectroscopy of complex systems. ork describes developments of chromophore-specific quantum chemical methods, which focus on relevant substructures without sacrificing the view on the entire system. A subsystem density-functional theory approach is analyzed that employs a real-space partitioning of the electron density for the description of complex aggregates in terms of simple fragments. This approach can be used as a chromophore-specific embedding method and allows for efficient and accurate analyses of environmental effects. However, it fails for phenomena caused by a collective response of an aggregate of molecules. The limitations of this embedding scheme can be overcome by a general subsystem approach to time-dependent density functional theory, which easily relates to phenomenological theories such as excitonic coupling models. nce Raman spectroscopy can be used to probe local excited states in larger molecules and is thus intrinsically chromophore-specific. It is shown that well-known approximations for resonance Raman calculations can efficiently be used with time-dependent density-functional theory methods to study photochemical and photophysical processes in large molecules such as artificial photosynthetic systems. Intensity-driven approaches to resonance Raman calculations can exploit the selectivity observed in experiments for an iterative determination of high-intensity spectral features. Applications of such schemes to biochemical building blocks are discussed.