Component 3: What is special about B3LYP/6‑31G*?
B3LYP31–34 is undoubtedly the most used and cited functional,121 and it is the de facto standard for calculations involving organic molecules and even for teaching purposes.122–126 B3LYP is the acronym for B88 exchange31 with Lee, Yang and Parr correlation32 in the 3-parameter scheme of Becke.33 B3LYP was proposed by Frisch et al. in 1994 for a study involving circular dichroism spectra of different molecules,34 and it is based on a variation of the B3PW91 functional of Becke.31,33,127,128 In B3LYP, the PW91 correlation functional127,128 is replaced by the LYP functional, but the remaining parts are unchanged (B88 exchange and the 3-parameter scheme). The 6‑31G* basis set is a double-ζ basis set of the Pople family,94–102 and it includes a very manageable amount of basis functions. This method rose to popularity in the mid 1990s, but since the mid 2000s many failures have been documented in the specialized literature.83,85,129–135 Unfortunately though, B3LYP/6‑31G* has become almost a synonym for DFT calculation itself in much of the broader audience (non-specialistic) literature.136–140 The educational literature is not exempted, as this method is applied without any formal justification, nor critical evaluation of the results even for reactions that are known to be problematic.122–126,139,141 We specifically designed experiment 7 to give first-hand experience on the limitations of B3LYP/6-31G*. Selected results are reported in Figure 2 , while full results are given in the SI. Instead of looking for systems where B3LYP/6‑31G* is successful, we encourage practitioners—as others did before us131—to move away from it and to seek better methods for potentially problematic cases. As a guide, all other hybrid GGA functionals are computationally as expensive as B3LYP (they are on the same rung of Jacob’s Ladder), while modern basis sets have similar number of basis functions as 6-31G* (def2-SV(P)107 has even less).