When trying to decide which species belongs in a kinetic model and which can be safely omitted, Fig. , it would be very helpful to have reliable numbers for the rate coefficients of each potential reaction. However, in many systems there are known to be hundreds or thousands of important species, and most species can react rapidly with all the radicals and some of the stable molecules, often in multiple ways. So today one would like to have reliable k's for perhaps a million reactions, and in the future even more. Probably in coming decades, as both computational chemistry methods and computer hardware improve, it might become possible to compute a million reactions of medium size molecules accurately from first principles, but today it is not practical. So today we instead need to rely on estimates, as discussed next. correlations could be cast so that the handful of adjustable parameters could be unambiguously determined by linear least-squares fitting to a small number of data (though sometimes even these linear least-squares calculations ran into ill-conditioning problems). Several of these estimation methods involve first identifying which functional groups are in the molecules, or which bonds are changing during the reaction, and then using the appropriate linear correlation(s), shown schematically in Fig. . In modern parlance, that first step converts a molecule or reaction into a "fingerprint" vector, and the second step converts the vector into the estimate of the quantity of interest (e.g log(k) or log(Keq)).