First principles simulations of electronic spectroscopy for larger molecules

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To gain a detailed understanding of molecular structures involved in complicated reactions, e.g. in a biological context, it is vital to be able to interpret a variety of spectroscopic measurements. For systems involving multiple transition metal atoms, viable spectroscopies include Resonance Raman, UV-VIS absorption, circular dichroism, magnetic circular dichroism, electron paramagnetic resonance, X-Ray near edge absorption, X-Ray emission, Mossbauer, XPS, NMR, and various time-resolved pump-probe techniques. The interpretation of spectra is a complicated process, and is currently often based on matching similarities to known spectra, and the use of simple models. The interpretation of the spectra, and perhaps even more importantly, the (structural) characterization of the system implied by the spectroscopy, can be greatly aided by reliable first principles theoretical simulations, the subject of this proposal. In the past we have developed technology to accurately simulate electronic spectroscopy for small molecules. The key ingredients involve the extraction of so-called vibronic model parameters from accurate coupled cluster type electronic structure calculations, and the evolution of quantum nuclear motion on multiple surfaces using non-adiabatic quantum wave packet dynamics. The key challenge is to generalize our methodologies to be applicable to larger systems. We plan to incorporate our methodology in the state-of-the-art ORCA electronic structure program, developed in the group of Frank Neese at MPI Mulheim an der Ruhr, and to capitalize on local correlation, in particular pair natural orbital techniques, which have been applied to systems as large as 50-100 atoms. In addition ORCA allows us to conveniently incorporate relativistic effects, in particular spin-orbit coupling, and this is a vital ingredient to simulate important spectroscopic properties. ORCA will be generalized to compute vibronic model parameters, while simulation of non-adiabatic dynamics is pursued employing the widely used Multiconfiguration Time-Dependent Hartree (MCTDH) package, developed in the Meyer group in Heidelberg, Germany.