FROM CONCEPTS TO ALGORITHMS FOR THE CHARACTERIZATION OF REACTION-MECHANISMS - H2CS AS A CASE-STUDY

All the stationary points on the So surface of H2CS and their quadrati c force fields have been determined by correlated methods using large multiply polarized basis sets. Complete quartic force fields of all st ationary points and reaction paths connecting each pair of minima thro ugh a first-order saddle point have been obtained at the MP2 level usi ng smaller polarized basis sets. These data allow the computation of r eliable isomerization and fragmentation rates by semiclassical express ions which take into account tunneling as well as curvature and mode-m ode couplings. Further insight into the characteristics of the differe nt reaction channels has been gained from the parameters of the so-cal led reaction path Hamiltonian (RPH) both in adiabatic and diabatic rep resentations. The bookkeeping and processing of the large body of data involved in this study has been possible through the development of a package which is able to perform anharmonic and reaction path computa tions. The results are discussed with special reference to the H2CO sy stem and to the interplay of potential, kinetic, and statistic effects in determining reaction mechanisms and rates.