  In astrochemistry, computational methods play a crucial role in addressing
fundamental astronomical questions. Interstellar molecules profoundly influence
the chemistry and physics of the interstellar medium (ISM), playing pivotal
roles in planet formation and the emergence of life. Understanding their
chemistry relies on theoretical approaches such as Density Functional Theory
(DFT) and post-Hartree-Fock methods, which are essential for exploring pathways
to molecular complexity and determining their interstellar abundances. Various
theoretical methods investigate the formation of interstellar molecules in both
gaseous and solid states. Molecules in interstellar space may originate from
bottom-up processes (building up from CO molecules) or top-down processes
(polycyclic aromatic hydrocarbon fragmentation). Here, we present a journey of
theoretical investigations aimed at studying the reactivity of interstellar
molecules in space.
