In this thesis, collisions involving the H2 reaction complex are studied theoretically and the
processes considered are mutual neutralization, double charge transfer, associative ionization,
dissociative recombination and resonant ion-pair formation. These processes are examples of
reactions that involve several excited states and where the Born-Oppenheimer approximation
is not applicable. The H2 system is one of the simplest examples of a diatomic molecule and it
thus provides an optimal system on which theory can be tested. The purpose of the present
work is to develop a theoretical model in which the cross sections of all of the above
processes can be computed, using the same set of potential curves and couplings. This
theoretical model is not limited to H2 and may serve as a basis for which more complicated
systems can be studied. In this work, calculations that include effects such as rotational
couplings and autoionization are carried out on H+ + H− mutual neutralization using this
model. These effects have not previously been considered in studies on mutual neutralization
for this system. Moreover, the theoretical model is applied in preliminary calculations on
double charge transfer, associative ionization and dissociative recombination and future
developments are discussed.