Extending the applicability of Many-Body Dispersion (MBD) to large-scale systems: DFTB+MBD

The inclusion of van der Waals (vdW) dispersion interactions in electronic-structure calculations has significantly extended the applicability of DFT. However, the accessible system sizes with DFT+vdW remain small for many practically relevant applications. On the other hand, Density-Functional Tight-Binding (DFTB) is an electronic-structure method of choice for systems with several 1000s of atoms. A severe drawback of DFTB and other semi-empirical methods, however, is the missing direct access to atomic polarizabilities, required for most common \textit{ab initio} vdW models. Here, we present a novel approach to derive accurate effective polarizabilities directly from the atom-centered basis set representation of the density-matrix [1]. This enables the use of sophisticated dispersion models such as the Many-Body Dispersion (MBD) scheme [2] in conjunction with most electronic-structure methods including both DFT and DFTB, thus allowing the investigation of many-body effects in large-scale systems. We exemplify the viability of such a combined approach by adressing the impact of many-body dispersion on the solvation effect in biomolecular systems with DFTB+MBD. [1] M. St\"{o}hr et al., J. Chem. Phys. 144, 151101, 2016; [2] A. Tkatchenko et al., Phys. Rev. Lett. 108, 236402, 2012.