DFTB+ ecosystem for modern Fortran software development: tools and interfaces

The DFTB+ code is a semiempirical density-functional-based code that is licenced as LGPL3. It supports density-functional based tight binding, the xTB hamiltonians and externally provided atomistic electronic models. As part of its wider project (www.dftbplus.org) we have developed several tools, under BSD or LGPL licenses, that are of potential interest to other Fortran-based codes. This presentation will describe our approaches to libraries and free standing tools for: meta-programming, interfacing with other codes, wrapping external libraries, and unit testing. Most of these developments are briefly described in our 2025 J. Phys. Chem. paper (in-press). But planning for the next phase of the DFTB+ code is underway: we will outline some of the gaps in our ecosystem and plans to fill them (HDF5 support, closer integration with the TBMaLT toolkit and other machine learning tools, improvements to our regression testing framework and further API extensions).