At the outset of the 21st century, and after eighty years of the advent of quantum mechanics, theoretical studies on small species still raise a lot of interest as foundations upon which research on methods for treatment of general systems is based. It is often believed that the gap which separates the exactly treatable models from the experimental results works against multidisciplinary character of research that is sought in modern science. However, in the case of quantum dots this common belief does not reflect reality, as these systems can have their structures theoretically analyzed in an exhaustive way and at the same time they are of great experimental relevance. Harmonium atoms are an especial kind of 3D quantum dots in which the central Coulombic potential of nuclear attraction is replaced by a harmonic one. For a small number of atoms these systems are analytically solvable or quasi-solvable. In the recent years there has been a renewed interest in harmonium atoms, particularly in the so-called Wigner molecules. In this project we study several harmonically confined clusters, and few-electron harmonium atoms with the aim to understand the electronic structure and the energetics of these species, especially at strong correlation limit (Wigner molecules) in order to provide guidance for future experimental research on quantum dots and assessment of their usefulness as benchmarks for DFT, DMFT and other electronic structure methods.