It has become possible in recent years to compute state-to-state reaction crosssections and rate constants from first principles for a few elementary chemical reactions and for a few energy transfer processes. We illustrate the state-of-the art using examples of results obtained from our own laboratory.

We report the results of a three-dimensional time-dependent quantum mechanical study of the reaction He + H₂⁺ (v = 0,1 ) → HeH⁺ + H at 〈Eintrans 〉 = 1.0 eV, which reproduces clearly the vibrational enhancement for the system. In addition, preliminary results for He + HD⁺ (v = 1–3) suggest the preferential formation of HeD⁺ over HeH⁺ in the products.

The reaction of ground-state atomic oxygen [O(${}^3P_2$)] with methyl, ethyl, 𝑛-propyl and isopropyl radicals has been studied using the density functional method and the complete basis set model. The energies of the reactants, products, reaction intermediates and various transition states as well as the reaction enthalpies have been computed. The possible product channels and the reaction pathways are identified in each case. In the case of methyl radical the minimum energy reaction pathway leads to the products CO + H₂ + H. In the case of ethyl radical the most facile pathway leads to the products, methanal + CH₃ radical. For propyl radical (𝑛- and iso^-), the minimum energy reaction pathway would lead to the channel containing ethanal + methyl radical.

The structure and stability of spiro-cyclic water clusters containing up to 32 water molecules have been investigated at different levels of theory. Although there exist minima lower in energy than these spiro-cyclic clusters, calculations at the Hartree-Fock level, density functional theory using B3LYP parametrization and second order Møller-Plesset perturbation theory using 6-31G^∗ and 6-311++G^∗∗ basis sets show that they are stable in their own right. Vibrational frequency calculations and atoms-inmolecules analysis of the electron density map confirm the robustness of these hydrogen bonded clusters.