Ab initio quantum chemistry has evolved into a matured field, with tools capable of providing reliable predictions of molecular properties. Notwithstanding the progress, much effort has been investigated in developing methodologies that address very challenging systems. Herein, three studies are presented, including both the application and development of electronic structure theory. First, a study on nonahydridorhenate dianion and molecular potassium and sodium rhenium hydride is reported using coupled cluster theory. Second, the potential energy surface of the methylene internal rotation is carefully investigated for n-propyl radical. The fundamental vibrational frequencies are predicted using second-order vibrational perturbation theory, and energy levels of the methylene torsional motion are also determined. Third, a multireference generalization of the driven similarity renormalization group is introduced. A perturbative analysis of the corresponding equations leads to an efficient and intruder-free multireference perturbation theory.