A molecular potential energy surface (PES) describes the energy of a chemical system as a funct ion of its geometry and is an essential ingredient for many applications of computational chemistry including spectroscopy and dynamics. Part of this talk describes the development of a PES generator (software code) which uses paral lel processing on High-Per formance Computing (HPC) clusters to const ruct PESs automat ical ly. Thousands of ab initio data are computed at geometries chosen by the algorithm and fi t to a functional form. The electronic structure of molecules is dif ficult to describe continuously across global PESs since i t changes qualitativel y as bonds are formed and broken along reaction coordinates. I will discuss the development of a high-level ab initio method (GDW-MRCI) designed to allow the electronic wavefunct ion to smoothly evolve across the PES and provide an accurate and balanced descript ion of the various regions. These methods are combined to study a number of applications f rom the areas of atmospheric, combustion and interstellar chemistry.