Full quantum mechanical modeling of protonated water clusters

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Over the past two years we have made very substantial progress towards the two main work packages set for the initial proposal: (1) automatic and on-the-fly determination of optimal tree structures in the context of the multi-layer multi-configurational time-dependent Hartree method; (2) full quantum mechanical modeling of the spectroscopy and dynamics of the extended Zundel (EZ) cation. The latter has been described in full dimensionality (51 D) using polyspherical coordinates and an exact kinetic energy operator, and on a very accurate neural-network potential energy surface provided by the group of Prof. D. Marx (U. Bochum). Specifically, we have developed and implemented tools for the automatic construction of wavefunction tensor trees, and we have been able to calculate good quality infrared spectra of the EZ system. The main aim of the extension project is still to push the boundaries for the full quantum dynamical description of flexible and highly correlated molecules and clusters through new methodological developments based on dynamical tensor-networks and the multi-configuration time-dependent Hartree Ansatz and to apply them to the full quantum mechanical description of the EZ cation in the gas phase and in solution. We have come a long way in the desired direction. In the extension period weplan to: (2.1) study in detail the connection between the Zundel and EZ spectra, and from this understand the microsolvation of the excess proton fully quantum-mecanically. (2.2) We plan to study the embedding of the EZ cation in external fields obtained from simulated environments, and thus take definite steps towards a full quantum characterization of the solvated proton in solution. Advancement in those areas will be important for the further understanding of solvated protons, protonated water clusters, and proton-relay networks fully quantum mechanically. Through the methodological developments of this project, we are pushing the state-of-the-art and enabling the full quantum simulation of large and flexible systems beyond current capabilities. Last but not least, all developments are being made immediately available to the community through the worldwide used Heidelberg MCTDH software package.