In the first study, Interacting Quantum Atoms (IQA) and Interacting Quantum Fragments (IQF) analyses were used to study F_3 C-X?NH_3 (X= Cl and Br) model complexes in order to determine the origin of halogen bond directionality. IQA allows for the calculation of intra- and interatomic classical and exchange-correlation energies. The Relative Energy Gradient (REG) method is also applied to rank the IQA energies and reveal which energy contributions best describe the total behavior of the system. Indeed, all the pairwise interactions and atomic self-energies are angularly dependent; some terms favor the linear structure and some tend toward nonlinear arrangements. Furthermore, the REG values revealed that the contribution of the X…N interaction to the total behavior of the system is small. Finally, IQF calculations followed by REG analysis revealed the importance of the self-energy of the fragments. In the second study, different computational methods were used to investigate the nature of interaction in the fluorine-containing complexes in which the fluorine atom acts as a Lewis acid and forms a non-covalent bond with the ammonia (Lewis base). The fluorine non-covalent interactions are somewhat different from the well-known halogen bonds of chlorine, bromine, and iodine. The halogen bonds of Cl and Br containing molecules are directional and the C-X-N angle tends to be linear. In contrast, the fluorine interaction is not directional; its interaction energy profile is flat over a wide angular range (from 180° to 140°). However, for the angles less than 130°, the energy curve showed a clear angular dependence and the F-N interaction becomes stronger as the C-F-N angle decreases. It seems that at the tighter angles, a tetrel bonded complex is preferred. In the third study, various complexes containing F?F interaction were studied to find the nature of the non-covalent fluorine-fluorine interactions. Since the charge, electrostatic potential and electron density distribution of the interacting fluorine atoms are identical, their interaction cannot be regarded as a Lewis acid-base interaction. However, there is a bond path and hence a bond critical point in the F…F interactions. It is noted that, although the fluorine atoms are connected by bond paths, IQA analysis showed that the F…F interactions are repulsive in nature and destabilize the complexes. Instead, the secondary interactions (i.e. the interactions between not connected atoms) are the driving force for the complex formation. IQF analysis presented that, the quantum inter-fragment interaction plays an important role in the stabilization of the complexes. Hence, the NBO and NBCP analyses and NAdO orbitals were studied to find the origin of the electron delocalization between the monomers. Fluorine lone pair NBOs contributes noticeably to electron density at the position of the BCP(F...F), although the C-F bonding NBOs have a non-negligible contribution in ?_BCP. In addition, the NAdO orbitals which localized on the n-centered monomers have key contribution to the DI value between monomers