Understanding excited-state properties of molecules in solution using embedded, accurate ab-initio wave functions

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The adequate description of solvation phenomena is the key to successful computational modeling of molecular properties in solution.The project aims at understanding excited-state properties of molecules in complex environments by dividing the system into subsystems and using ab-initio wave-function methods.When investigating a molecular complex in a non-trivial environment with ab-initio quantum chemistry, one faces different challenges at the same time. An obvious problem is the steep scaling of the available (wave-function) methods so that in practice only a very limited number of atoms can be treated. A more subtle problem with increasing system size is the amount of states and degrees of freedom arising due to the number of molecules involved, so that, independently of the method used, an explicit treatment of all molecules in a supermolecular calculation makes the analysis of chemically motivated subunits often too hard.One useful ansatz is given by embedding methods, which divide the supersystem into smaller subunits, so that a very limited number of states are left which are by definition assigned to a certain molecule, and the scaling problem is significantly reduced. Frozen-density embedding (FDE) has proven to be an efficient approach to divide a complex consisting of several molecules, with all subsystems treated using ab-initio methods. One of the main features of this method is to allow for a systematic, consistent and rigorous derivation for the most common molecular properties with wave-function methods, avoiding special-case treatments for some properties, or the need to reparameterize semi-empirical parameters.In this project, we shall develop analytical nuclear gradients for an approximated coupled-cluster singles and doubles (RI-CC2) FDE for both ground and excited states. The new method can be used to investigate the origin of excitations and their influence on the (excited-state) geometry in complexes with solvation shells, while it is possible to discriminate between significantly shifted local excitations and ''true'' super-system effects, such as inter-fragment charge-transfer excitations. The main applications of this project are seen in small molecular complexes surrounded by explicit solvation molecules for which the RI-CC2 method is applicable. This yields a large variety of interesting complexes, ranging from for instance deoxyribonucleic acid (DNA) dimers to the benzene dimer, i.e. from hydrogen bonds to van-der Waals interactions. Particularly, systems of interest are excimers in solution, where an excited-state geometry optimization is significantly more efficient than a single-point scan if more than one degree of freedom needs to be taken into account.