Embedded wavefunctions for 2D and 3D periodic molecular systems

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Despite the development of different approaches, the use of correlated wave functions in periodic sys- tems is still a challenge. The present application aims to enable accurate description of local properties in periodic molecular systems, such as local defects in molecular crystals or hopping-based transport in or- ganic semiconductors, using correlated wavefunction methods. While wavefunction methods provide the required accuracy for dynamic or static correlation, the computational cost of conventional approaches scales significantly with system size, limiting in particular the application of wavefunction methods to molecular solids and liquids in practice. Frozen-density embedding (FDE) enables quasi-linear scaling with number of subsystems, greatly facilitating the study of local properties including defects. In the present application, embedded wavefunctions using a local Gaussian basis will be used in 2D and 3D periodic systems in combination with fast multipole methods for the long-range Coulomb contributions. Freeze-Thaw iterations are used to relax molecules in close proximity to perturbations or defects. The proposed approach allows in particular to avoid the periodic repetition of the defect, thus providing a fully relaxed self-consistent description of the bulk phase and surfaces using correlated wave functions. This approach is able to describe long-range electrostatic effects on e.g. local excited states, but in addition to simple electrostatic models it also ensures repulsion contributions due to the effective embedding poten- tial at short distances, so that both intramolecular and intermolecular effects are adequately accounted for. The newly developed methods are particularly applicable to organic semiconductor materials such as tetra-aza-peropyrenes (TAPPs), but can also be used to study local properties of solutes.