How Well Can Rmp4 Theory Treat Homolytic Fragmentations?

The observation that fourth-order restricted Moller-Plesset perturbation theory (RMP4) gives a satisfactory description of the He:+ potential curve up to the transition structure for dissociation has led to a general examination of the applicability of RMP4 theory to the study of homolytic fragmentation in dications. It appears that, in many cases, RMP4 energies do indeed provide a useful estimate of the barrier height impeding such fragmentations and that the barrier may be estimated economically by calculating RMP4 single-point energies on the UMP2 geometries of the equilibrium and transition structures. We propose the use of a new quantity, the A parameter, as an approximate measure of the rate of convergence of the RMP perturbation series and hence of the reliability of RMP4 energies.