Component 1: Which functional should I use?
Given the sheer amount of options available, a key issue in modern KS-DFT research is the choice of an approximate xc functional. This is not an easy task, as the accuracy and reliability of the results is dependent on the functional chosen to describe the chemical system. If one chooses a functional that is not suitable for a given chemical application, the results will not be reliable (in accordance with the old computer science adage: garbage in, garbage out). This does not mean that DFT fails as a whole, but rather that the specific approximation is inadequate for the chemical problem under investigation. In this context, we may think of the functional (and not of DFT) as a tool needed to perform a specific task. If the tool is not adequate, we should choose a different tool, or a different task. If we need to put a nail in the wall, we will pick a hammer, regardless of the number of screwdrivers we have at our disposal. As Kathrin Hopmann recently advised: “Do not use DFT for mechanisms it cannot handle”.70 Many topics at the center of the last decades of DFT research, such as dispersion corrections,71–76 broadly applicable meta -GGA functionals,36,37,40,43,44 and functionals for strong correlation,36,42,69,77–81 have been introduced and studied to provide better and more reliable approximations. Several of these new advances have been facilitated by the introduction of large databases of accurate data of broad chemical interest.25,26,36,43,69,82 The use of databases outside the theoretical chemistry community is encouraged as they are the primary source for suggestions when choosing the xcfunctional for a specific application. It is good practice to compare the specific reaction under investigation with a similar one in a computational database before choosing a functional. In addition, databases can also give an estimate of the expected accuracy of the results over a broad spectrum of chemical properties. The reactions we will analyze in this tutorial come from three important collections of chemical data, namely MGCDB84,26GMTKN55,25 and Minnesota 2015.36,43,69
Table 1. Structure of the laboratory notebook and corresponding take-home message.