In this thesis, theoretical ab initio treatments of two-body molecular collision reactions are studied, having in common that the interaction region including all coupling mechanisms driving the reaction amounts to a molecular description. The main goal is to gain an understanding in the underlying coupling mechanisms involved in these reactions.The thesis is divided into three projects. In project one, mutual neutralization in collisions of Na⁺ + I⁻, C⁺ + Cl⁻ and H⁺+H⁻ are studied, with an emphasis on the inclusion of spin-orbit and/or rotational couplings which are most often neglected for in mutual neutralization. Scattering quantities are computed ab initio and compared to approximative models and experimental results. In project two, the problem of asymptotic non-adiabatic couplings is studied. Specifically, the inclusion of higher order terms in the reprojection method is shown to give a much faster convergence of the relevant scattering cross section. The method is here applied to mutual neutralization in H⁺+H⁻collisions and inelastic scattering in Li+Na and H+H collisions. In project three, a generalized pseudo Jahn-Teller model is introduced an applied to electronic resonant states of H₃. Model parameters are extracted using electron scattering calculations resultingin a non-Hermitian Hamiltonian describing the system. The topology of the resulting complex adiabatic potential energy surfaces, including complex conical intersections and non-Hermitian degeneracies, are furthermore studied and classified.

In this thesis, reactive scattering processes involving the H₂ reaction complex are studied ab initio and fully quantum mechanically. These processes have in common that they involve highly excited electronic states, which could be either bound or resonant. Non-adiabatic couplings, which can be significant both at small and large internuclear distances, need to be included to account for the interaction between the bound electronic states. In addition, the electronic resonant states interact with the ionization continuum at small internuclear distances, which may cause the collision complex to autoionize. In this work, a model is developed which incorporates these different coupling mechanisms. By introducing a quasidiabatic model at small internuclear distances, resonant states and couplings to the ionization continuum are incorporated. The quasidiabatic model is combined with a strict diabatic description, which rigorously incorporates non-adiabatic couplings among the bound electronic states. Nuclear dynamics are solved for using a close-coupling approach in a strict diabatic representation, where a non-local complex potential is included to account for loss into the ionization continuum. With this model, various reactive scattering processes can systematically be studied using the same set of potential energy curves and couplings. The model is applied in studies of H⁺+H^- mutual neutralization, H(1s)+H(ns) and H⁺+H^- associative ionization as well as dissociative recombination and resonant ion-pair formation in electron collisions with HD⁺. Cross sections and branching ratios are compared with results from previous experiments and theoretical studies.