Too continuous to continue? Multiple electronic surfaces and derivatives

The current document includes six chapters, which are published, accepted, or prepared for submission in various journals. Chapter 2 introduces a study on the triplet surface of O+C2H4. Chapter 3 presents a study on the singlet and triplet surfaces of O+C2H4 with a focus on biradical species. Chapter 4 continues the previous study with a more extensive study on the energetically lowest-lying singlet surface of O+C2H4. Chapters 2-4 rely on a several multireference methods to model ground and excited states and on some single-reference methods. Chapter 5 presents a multireference study on SiCH4 and butadiene with ORMAS energy, first-order nuclear derivative, and first-order nuclear derivative coupling contours, which characterize whether or not the ORMAS approximation effects correct analytical derivatives with seemingly smooth energies over a range of geometries. Chapter 6 presents the interface of NEWTON-X (a dynamics driver program) and GAMESS (an electronic structure suite) along with a dynamics study on CNH4+ that demonstrates the possible effects of the ORMAS approximation on product distributions.