This motivated the development of composite Kohn-Sham-DFT methods -for example, from the 3c-family B97-3c or r 2 SCAN-3c 620,621 at the (meta)GGA level. The sought-after compromise between computational effort and accuracy is achieved here by applying tailored, medium-sized atom-centred AO basis sets on top of standard or slightly modified DFAs and adding appropriate atom pairwise potentials to account for dispersion and basis-set incompleteness. The recently proposed r SCAN-3c method outperforms some hybrid-DFT/QZ approaches for reaction and conformational energies as well as for noncovalent interactions at a speed-up of two to three orders of magnitude. 621 (4.2.10) Köster : For QM/MM MD applications, Kohn-Sham DFT with density fitting or auxiliary DFT (ADFT) are very promising QM methods. In combination with DFT-optimized basis sets and automatically generated auxiliary functions, ADFT Born-Oppenheimer MD simulations on the nanosecond timescale are possible. These calculations permit the simulation of finitesystem melting, to determine the corresponding melting temperatures and latent heats. The extension of these calculations to QM/MM models will allow the simulation of finite-system phase transitions in MM environments within the NVT and NPT ensembles.