Emperical potential energy surfacesA strong argument can be made that the most preductive theory or model is one that can be calibrated to experimental results and can be refined and extended as new, perhaps unexpected results become available. Many of the empirical schemes for constructing potential energy surfaces fall into this category, since they contain parameters that can be adjusted to fit surface to experimental data, such as activation energies. In principle such fitting can also be extended to reproduce observations of apportionment of energy among translational, vibrational, and rotational modes in products or the effect of different kinds of reactant energy on observed cross sections for reactions. However, the latter cases require extensive trajectory calculations for each choice of parameters to obtain predicted energy distributions of cross sections, and they also require experimental data that are available in only a small but growing number of cases. A less expensive and more common approach is to use adjustable parameters in an empirical surface to vary common approach is to use adjustable parameters in an empirical surface to vary the positions and thickness of the barrier or the contours of the valleys and, by comparing trajectories on the various surfaces, explore how the overall shape of the surface influences cross sections and hence rate constants. Adjustable parameters can also be used to fit an empirical surface to the results of and ab intio calculation. This is important because ab intio calculations yield the potential energy only at a limited number of discrete points on the surface, while trajectory calculations require values of potential energy and its derivatives at arbitrarily selected points.Some empirical surfaces are based entirely on data from outside the field of kinetics and contain no adjustable parameters. An example is provided by the work of Murrell on HCN. The potential energy is a function of the three internuclear distancesThe diatomic potemtial energy The diatomic potential energy