In this thesis the helium(He) nanoclusters in the size range of 2 to 100 He atoms were considered. The structures belong to different symmetry point groups. Also the SF 6 molecule was selected and put inside the He nanoclusters including 20, 40 and 60 He atoms. The structures were optimized employing DFT methods. The He 2 to He 29 nanoclusters were used to study the absorption spectra at the CIS/6-311+G(2d,p) level of theory. Primarily it was foun d that the selected basis set must be augmented so the extra diffuse functions were employed. Then the excitation energies were calculated in the enegy range of 21 -24 eV and the absorption spectra were plotted which were consisted of three distinct features. It was found that the first excitation energies were increased as the result of clustering so that all the absorption energies were more than the related energy in atomic He(21.33 eV). The excitation energies of He 20 and He 29 were compared with the experimental spectra. I n the experimental spectra , the general shape of the spectra are nearly independant of the size of the cluster and the intensity of the first peak were increased as the size of the cluster increased. This was approved by the theoretical calculations. The photoelectron spectra of He structures in the size range of 2 to 100 He atoms were calculated using SAC-CI/6-31+G(3df) level of theory. It was deduced that the first ionization energies were independant of the size in the range of n 50. The variation of the calculated first ionization energies versus the size were fitted into an extended Morse potential equation. It was found that the calculated separation of the first and the last ionization energies and the sepatation of the first and the last peak in the spectra were independant of the size in the range of n 50. The photoelectron spectra were compared to each other and compared to experimental spectrum and the ionization bands were assigned using NBO methods. The spectroscopic properties of SF 6 in the He nanoclusters ( including the p hotoelctron spectrum, the absorption spectrum and IR and Raman spectra of doped SF 6 and the effect of the size on these spectroscopic properties) were studied. By the comparison of the photoelectron and absorption spectra of SF 6 @He n with the related spectra of free SF 6 , it was found that the intermolecular interaction in the smaller structures affect the spectra of SF 6 . By the increase of the size, the effect of intermolecular interaction reduced on the the spectra so that the photoelctron and absorption spectra of SF 6 in the bigger clusters of He, resemble to the related spectra of free SF 6 . For the first time, in this thesis the intermolecular interaction effect and t he deformation effect on the spectroscopic properties were separatedly studied. It was found that the intermolecular interaction effect on the absorption spectrum of SF 6 was more than that on its photoelectron spectrum. By the increase of the size the intermolecular interaction effect decreases.