Also molecular implementations of sDFT can be extended towards to treatment of condensed-phase systems. Höfener and coworkers have developed a scheme that imposes periodic boundary conditions in one dimension. To this end, an embedding potential is generated by periodically repeating the electron density of the active molecular subsystem, which is in turn used to embed this active subsystem into the environment of its own periodic images. This has been applied to calculate electron attachment and ionization as well as excitation energies in one-dimensional stacks of organic semiconductors . 3D quasi-periodic calculations have been achieved in terms of so-called relax-copy-cycles for molecular crystals . During the relax step in these cycles, the orbitals of the molecules in an inner unit cell are determined, embedded in a guess electron density for the molecules in surrounding unit cells. Afterwards, the density in the surrounding unit cells is updated by copying the density of the innermost cell. This is repeated until the orbitals/densities of the molecules in the central unit cell do not change anymore. This strategy has been applied in Ref. 12 for spin-density calculations of clusters containing several thousand QM atoms (see also Section 6.5).