Multireference approaches for excited states of molecules

Understanding the properties of electronically excited states is a challenging task that becomes increasingly important for numerous applications in chemistry, molecular physics, molecular biology, and materials science. A substantial impact is exerted by the fascinating progress in time-resolved spectroscopy, which leads to a strongly growing demand for theoretical methods to describe the characteristic features of excited states accurately. Whereas for electronic ground state problems of stable molecules the quantum chemical methodology is now so well developed that informed nonexperts can use it efficiently, the situation is entirely different concerning the investigation of excited states. This review is devoted to a specific class of approaches, usually denoted as multireference (MR) methods, the generality of which is needed for solving many spectroscopic or photodynamical problems. However, the understanding and proper application of these MR methods is often found to be difficult due to their complexity and their computational cost. The purpose of this review is to provide an overview of the most important facts about the different theoretical approaches available and to present by means of a collection of characteristic examples useful information, which can guide the reader in performing their own applications

Beschreibung angeregter Molekülzustände in komplex strukturierter Umgebung durch einen effizienten, individuell selektierenden MRCI-Algorithmus gekoppelt an ein molekularmechanisches Kraftfeld

Atomistic theoretical descriptions of thermal chemical recations in complex environments are well achieved by combined quantum and molecular mechanical (QM/MM) methods. The goal of this work is to extend such techniques to enable the description of photochemical reactions and to carry out case studies subsequently. In order to tackle the enormous computational demand due to the involvement of excited electronic states, we (i) largely speed up the individually selecting multi-refernce configuration interaction (IS/MRCI) scheme by Tavan and Schulten (1980) by a new grahical algorithm and (ii) take use of recently developed semimempirical valence shell models (Thiel 1997) which are well suited for excited electronic states. The efficiency and accuracy of the resulting IS/MRCI-algorithm is demonstrated by its application to the first electronic excited states of butadiene. The new QM/MM method is used to calculate absorption energies along a molecular dynamics trajectory of a small Schiff base in isotonic solution.Für die theoretische Beschreibung thermochemischer Reaktionen in komplexer Umgebung auf atomarem Niveau ist die Kombination aus quanten- und molekülmechanischen Verfahren (QM/MM) bereits etabliert. Ziel dieser Arbeit ist, für die Beschreibung photochemischer Prozesse ebenso ein QM/MM-Verfahren zu entwickeln und es exemplarisch zu validieren. Dem extremem Rechenaufwand bei der QM-Beschreibung der nun auftretenden elektronisch angeregten Molekülzustände wird hier begegnet durch (i) Beschleunigung der individuell selektierenden Multireferenz-Konfigurationswechselwirkungs (IS/MRCI)- Methode von Tavan und Schulten (1980) mittles eines neuen Graphenalgorithmus und (ii) den Einsatz von neuen semiempirischen Valenzschalenmodellen (Thiel 1997) mit besonderer Eignung für angeregte elektronische Zustände. Die Effizienz und Genauigkeit des neuen IS/MRCI-Algorithmus wird anhand der Beschreibung der ersten angeregten Zustände von Butadien demonstriert. Das neue QM/MM-Verfahren findet Anwendung bei der Berechnung der Anregungsenergien entlang einer molekulardynamischen Trajektorie einer kleinen Schiff-Base in isotonischer Lösung