(3.3.3) Gill : The development of a small number of data sets of unimpeachable experimental data, against which the predictions of new DFAs can be compared objectively and reproducibly, is essential. Both the data sets and the software used to evaluate DFA performance should be freely accessible and entirely transparent. It was the publication of such comparisons in the early 1990s that led to the widespread adoption of DFT in the chemical community. 335 (3.3.4) Gould : While not quite as robust as experimental data, we now have access to some impressively large quantumchemical benchmark sets (about 5000 energy differences in MGCDB84 and 1500 in GMTKN55 336 ) against which to validate DFAs. However, key overall quality metrics can be reproduced almost perfectly by about 150 entries of the original benchmark sets (less than 10%). This means that a large part of these sets contain redundant information. It is therefore important that validation protocols test DFAs across diverse physics and chemistry -we should not just assume that testing against more systems will automatically do a better job. New applications of statistical techniques may be required to develop robust validation protocols.