This dissertation is themed around multi-reference methods in quantum chemistry, focusing in particular on the spin-flip family of methods. Chapter 2 contains the details of an application

study on the excited states properties of 5-formylcytosine and 5-hydroxymethylcytosine,

two naturally occurring cytosine derivative bases. Chapter 3 investigates the Born-

Oppenheimer and beyond-Born-Oppenheimer stability of the experimentally elusive

ethylenedione molecule (OCCO). Chapter 4 introduces a new spin-correct spin-flip

quantum mechanical method based on the ORMAS-CI formalism by Ivanic, here labeled

SF-ORMAS, while Chapter 5 expands on SF-ORMAS with analytic nuclear gradients.

Lastly, Chapter 6 discusses the derivation and implementation of the non-adiabatic

couplings for the SF-ORMAS method.