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WIREs Comput Mol Sci
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Subsystem density‐functional theory

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Subsystem density‐functional theory (subsystem DFT) has developed into a powerful alternative to Kohn–Sham DFT for quantum chemical calculations of complex systems. It exploits the idea of representing the total electron density as a sum of subsystem densities. The optimum total density is found by minimizing the total energy with respect to each of the subsystem densities, which breaks down the electronic‐structure problem into effective subsystem problems. This enables calculations on large molecular aggregates and even (bio‐)polymers without system‐specific parameterizations. We provide a concise review of the underlying theory, typical approximations, and embedding approaches related to subsystem DFT such as frozen‐density embedding (FDE). Moreover, we discuss extensions and applications of subsystem DFT and FDE to molecular property calculations, excited states, and wave function in DFT embedding methods. Furthermore, we outline recent developments for reconstruction techniques of embedding potentials arising in subsystem DFT, and for using subsystem DFT to incorporate constraints into DFT calculations. This article is categorized under: Structure and Mechanism > Computational Materials Science
Schematic illustration of the different theoretical approaches available for large molecular systems, using the example of aminocoumarin C151 surrounded by twenty water molecules. (a) In conventional KS‐DFT, a single calculation is performed for the full system. (b) In subsystem DFT, the system is split into fragments, and the densities of all fragments are optimized self‐consistently. (c) In frozen‐density embedding theory, an approximate density is used for the environment, and only the density of the active subsystem is optimized.
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Structure of the model for the FMO complex (>7000 atoms), site energies of the bacteriochlorophyll subsystems, and most important excitonic couplings obtained in Ref .
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Analysis of the dipole moment of the HCNn chains from subsystem DFT (BP86/TZP/PW91k) calculations; (a) contributions of individual monomers in HCN7; (b) contributions of first and last monomer in HCNn chains () as well as largest monomer contribution.
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Average dipole moment of the HCNn systems () from subsystem DFT (BP86/TZP/PW91k) and KS‐DFT (BP86/TZP).
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Structure of the HCNn systems investigated here (example with ) and schematic representation of a certain step in the subsystem DFT calculation, in which the density of the second monomer is determined under the influence of an embedding potential derived from the density of the other systems (densities represented through isosurface plots).
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