During most of the 20 th century, it was difficult-to-impossible to accurately compute most gas-phase high-pressure-limit rate coefficients k∞(T) from first principles, and even more difficult to compute the effects of pressure or solvent on the rates. However, the development of density-functional theory and other quantum chemistry methods, coupled with the rapid advances in computer hardware, have now made DFT calculations routine, and . Rather sophisticated quantum chemistry calculations are now feasible for many reactions , and they are often quicker and easier than experimental measurements of similar accuracy. This game-changing advance is the reason that it is conceivable that predictions validated by inexpensive laboratory experiments, or in some cases even pure predictions, might be able to replace the conventional approach based on extensive pilot-plant experimentation shown in Fig. , while this was impossible in the 20 th century.