Over the course of the past couple of decades, Density Functional Theory (DFT) has grown into the workhorse of quantum chemistry and materials science, enabling a broad spectrum of applications. An excellent accuracy/cost trade-off can in principle be achieved with the help of DFT, under the condition that a suitable exchange-correlation functional approximation (F xc ) is selected. F xc are usually ranked using Jacob's ladder, where the most complex approximations can be found at the top. As a rule of thumb, functionals higher up the ladder tend to be more accurate, though this is most certainly not a universal rule that holds across application domains. As such, it is common practice in chemistry to determine the most suitable F xc for a given type of application based on a benchmarking against a limited set of either experimental or high-level wave-function computed, data points.