In the first part of this work different tautomers of 5-aminotetrazol in their first excited electronic states interacting with hydrazine in its ground electronic state are studied. Different theoretical methods including, time-dependent density functional theory (TD-DFT), configuration interaction singles (CIS) and complete active space self-consistent field (CASSCF) were used to optimize the structures in their first exited electronic states. In addition, the effects of different basis sets on the optimized structures were also studied. The vertical and adiabatic excitation energies of the considered isomers were also calculated and compared with isolated 5-aminotetrazol to see the effect of interaction with hydrazine in the first excitation energy. In the second part of this work, the absorption spectrum of seven complexes of hydrazinium 5-aminotetrazolate were calculated in both gas and water. The absorption spectra of the selected complexes were also calculated without considering the intermolecular interaction between 5-aminotetrazol and Hydrazine in the complex and compared with the calculated absorption spectra of isolated 5-aminotetrazol and Hydrazine in the gas phase. This comparison showed the effect of deformation on the electronic structure of 5-aminotetrazol and Hydrazine in the structure of hydrazinium 5-aminotetrazolate complex. In another part, the calculated spectrum of hydrazinium 5-aminotetrazolate was compared with the calculated spectrum of hydrazinium 5-aminotetrazolate without interaction between 5-aminotetrazol and hydrazine to investigate the effect of interaction between two fragments on the absorption spectrum of the complex. Similar studies were performed on the most stable structure of hydrazinium 5-aminotetrazolate in water and a different trend was seen for the effect of deformation and intermolecular interaction compared to the gas phase. In the last part of this work, the experimental photoelectron spectrum of a molecule was used as a tool for investigating how the different structures of the molecule, optimized at different levels of theory, are close to the real structure of molecule in gas phase. For this purpose, the different optimized structures of a molecule, optimized at different levels of theory, were used for calculating the ionization energies and photoelectron spectrum of molecule. It has been shown that the kind of the method used for the optimization has effect on the relative position and intensity of ionization bands and change the shape of the calculated photoelectron spectrum. In addition, it was observed for some cases that the number of calculated ionization bands in an energy region depends on the kind of method of used for the optimization of geometry of molecule. In addition, it has been proposed that the calculation of the ionization energies of a compound with different optimized geometries help us to reduce the error in the calculated photoelectron spectrum compared to experiment.