Much of our research over the years has the central theme of invoking a similarity transformation to simplify a large eigenvalue problem, followed by a relatively small diagonalization. This concept is joined to the use of normal ordering and Wick’s theorem to create powerful methods for both excited states and multireference problems in second quantization-based Quantum Chemistry. In our most recent work, the approach can also be used to calculate time-correlation functions to extract spectra for non-adiabatic problems based on vibronic Hamiltonians. In this presentation I will give a general overview of these various approaches.