One of the most challenging problems in the study of molecular electronic structure is the proper treatment of electron correlation. Methods such as Hartree-Fock theory do not fully account for it while Density Functional Theory does not allow a systematic improvement of the treatment. Full electron configuration interaction (FCI) does allow an exact treatment but its scaling does not allow routine application for any system of practical interest. Coupled cluster methods do allow a good description but there is also a plethora of less commonly known methods such as those based on constrained Hamiltonians or Geminals based methods including the Richardson-Gaudin method and seniority based methods. These will, in the coming years, be the subject of the research carried out in the group of the applicant.