I mentioned how challenging it is to solve the quantum mechanics of molecules. This problem stems from the appalling sizes of the Hilbert space in many-body systems. Chemists, and not only theoretical ones, also face another challenge: the size of the chemical space, which means., the number of molecules that could exist. Until a decade ago, our only way to explore new molecules was through our chemical intuition aided by similarity and heuristics rules. issue "Prediction Challenge: Cyclobutanone Photochemistry." See Ref. and references therein for theoretical data. Experimental results 1 from Ref. and 2 from Ref. were published after all simulations,