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Accurate theoretical studies of chemical systems with predictive power, relevant to experimental research, have only recently become possible by methodological and technological advances. In particular, multiscaling methods such as QM/MM and ONIOM have greatly contributed to understanding and enhanced control of chemical reactions in cases where a system can be partitioned into “active” and “bystander” regions . However, such partitioning schemes are obviously problematic when a system is either composed of regions that are highly correlated due to quantum correlation, or consists of a large number of chemically reactive fragments creating a complex chemical reaction environment. The former situation is encountered in metals and semiconductors, π−conjugated systems such as carbon nanomaterials and large photoactive molecules, whereas the latter often occurs in modern nanomaterials synthesis or chemical sputtering processes. For the theoretical study of such complex systems without obvious spatial partitioning, we found it effective to employ approximate density functional theory (DFT) methods based on density-functional tight-binding (DFTB) approximations. In the first part of this talk, we give a brief overview of our past quantum chemical studies of complex systems ranging from carbon nanostructure formation  over the simulation of vibrational  and optical  spectra to the study of molecular and electronic structures of COFs . In the second part of this talk, we lay out our vision for the fully quantum chemical study of enzyme reactions and bioimaging processes, and present preliminary results for the merger of the fragment molecular orbital (FMO) method with DFTB for free energy calculations.
 S. Irle and K. Morokuma, “Integrated Methods: Applications in Nanoscale Quantum Chemistry”, in: J. A. Schwarz, C. Contescu, and K. Putyera, Eds., Dekker Encyclopedia of Nanoscience and Nanotechnology, Marcel Dekker, New York, 2004.
 A. J. Page, Y. Ohta, Y. Okamoto, S. Irle, and K. Morokuma, “Mechanisms of Single-Walled Carbon Nanotube Nucleation, Growth and Healing Determined Using QM/MD Methods”, Acc. Chem. Res. 43, 1375-1385 (2010).
 D. V. Kazachkin, Y. Nishimura, H. A. Witek, S. Irle, and E. Borguet, “Dramatic reduction of IR vibrational cross-sections of molecules encapsulated in carbon nanotubes”, J. Am. Chem. Soc. 133, 8191-8198 (2011).
 C. Camacho, Th. Niehaus, K. Itami, S. Irle, “Origin of the size-dependent fluorescence blueshift in [n]cycloparaphenylenes”, Chem. Sci. 4, 187-195 (2013).
 X. Feng, M. A. Addicoat, S. Irle, A. Nagai, D. Jiang D. Jiang, “Control Crystallinity and Porosity of Covalent Organic Frameworks through Managing Interlayer Interactions Based on Self-Complementary π−Electronic Force”, J. Am. Chem. Soc. 135, 546-549 (2013).