C5 - Quantum chemistry and atomistic simulations of nucleic acid structures

Our high-level hierarchy of mainly quantum chemical methods (GFN-FF, GFNn-xTB, PTB, DFT-3c, DFT-D4, PNO-LCCSD(T)-F12) and multi-level workflows (CREST, CENSO) is further improved and extended by a newly developed general purpose tight binding method (GP3-xTB) and a fast composite range-separated hybrid DFT method (ωB97M-3c). In subproject one, thorough and comprehensive benchmarking for test sets specifically compiled for RNA structures and provided with very accurate coupled cluster theoretical reference energies enables us to establish the most suitable multilevel ONIUM-type QM/MM scheme. It is combined with an explicit solvation treatment and applied to model static and dynamic properties of nucleic acid structures in close collaboration with the experimental groups of the CRC. The second project part includes the computation of equilibrium structures, structure ensembles, their (free) energies as well as spectroscopic properties of various kinds (UV, CD/OR, IR, Raman, NMR, EPR). Furthermore, the response and dynamical changes of the respective RNA structures by external stimuli are studied, e.g., changes of the charge state by protonation. This requires automated protonation site (pK_a) as well as tautomerization state determinations for unnatural homologues closely related to the RNA systems studied by the experimental groups, but also the interpretation of measured IR, Raman, and NMR spectra for establishing common structure-property relationships. In the longer term, we also plan to establish a full quantum chemical calculation of resonance Raman spectra for large RNA systems using TB methods.