Explicitly Correlated Multireference Coupled-Cluster Study of Molecules and Cations with Diradical Character

Known and proposed helium chemistry embraces a large variety of small-size cationic species. Even though some of these species can be studied experimentally in the gas phase, the vast majority of contemporary studies are backed up or done entirely using high-level of theory calculations. The aim of this thesis is to present a theoretical introduction to ab initio computational chemistry from the basics (definition of the electronic problem, Born-Oppenheimer approximation, Hartree- Fock method) to the Brillouin-Wigner and Mukherjee Hilbert-space state-specific multireference coupled-cluster (MRCC) methods and their explicitly correlated versions. The thesis also contains a study of HeN2+ 2 using the mentioned MRCC methods. An important result has been, that Mukherjee's state-specific multireference coupled-cluster method with perturbative triexcitations Mk CCSD(Tu) gives results in very good agreement to conventional calculations (CASPT2 and MRCI) using a much smaller active space and smaller basis sets