"The in-medium similarity renormalization group (IMSRG) is an ab initio many-body method which features soft polynomial scaling with system size and a Hermitian framework to create Hamiltonians tailored for use with low-level approximations such as Hartree-Fock (HF) theory or the random phase approximation (RPA). The flexibility that comes with these characteristics has made the IMSRG a mainstay in contemporary nuclear structure theory. However, spectroscopy with IMSRG calculations has been limited to scalar observables in nuclei accessible with shell model machinery, where the IMSRG is used to construct effective valence-space interactions. In this thesis, we present two novel developments which have greatly extended the IMSRG's capability to perform spectroscopic calculations. First is the introduction of the equations-of-motion IMSRG (EOM-IMSRG), which uses an approximate, but systematically improvable diagonalization scheme in conjunction with the IMSRG to produce spectra and wave functions. The method does not suffer the model-space limitations of the shell model, but sacrifices some accuracy due to the approximate diagonalization. We benchmark this new method with the well established equations-of-motion coupled cluster and full configuration interaction methods, where we demonstrate that the method is indeed viable for closed-shell systems, encouraging expansion to open shells using a multireference formalism. We also introduce a perturbative framework to add systematic corrections to the EOM-IMSRG, showing results for closed shell nuclei and quantum dots. The second development is a generalized effective operator formalism for the IMSRG, capable of consistently evolving non-scalar operators relevant for electroweak transitions and moments. This general framework is applicable to both the EOM-IMSRG and the valence-space IMSRG approaches. We benchmark electromagnetic transition strengths and moments using both of these methods, also comparing with the quasi-exact no-core shell model and experiment when available. We demonstrate that consistent renormalization of observables is critical for precise computations with the IMSRG. We find that our methods perform well for transitions that are strongly single particle in nature, but for collective transitions involving many particles, we note that work remains to properly incorporate these effects in the IMSRG."--Pages ii-iii.
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