Applying accurate and modern ab-initio methods the electronic and optical properties of 2-aminopyrimidene-silver(I) semiconductors and ferroelectrics lithium niobate, lithium tantalate (LN and LT), and LN-LT mixed crystals has been investigated. Density functional theory, the quasi-particle framework, and the Bethe-Salpeter equation (BSE) provided the basis for the determination of the characteristics of the studied molecular and crystalline systems. Moreover, special issues related to the different material classes have been considered. During the investigation of the 2-aminopyrimidene-silver(I) semiconductors the calculated properties of the initial molecules have been compared to the properties of the corresponding solids, observing that, the molecular fingerprints are still visible in the optical absorption of the solids, albeit the transition from the molecular state to the solid state is accompanied by modifications of many electronic and optical properties. In this part of the thesis also the influence of the Tamm-Dancoff approximation on the optical absorption spectrum has been investigated by applying a special approach to image and evaluate the BSE matrix. The study of the ferroelectrics lithium niobate, lithium tantalate, and LN-LT mixed crystals involved the calculation of the LN band gap, which is often a matter of discussion in literature. A value of 5.4 eV for the LN band gap has been determined in the present work. The comparison of the optical absorption spectrum with experimental measurements shows that, the current value have to be close to the actual value of the LN band gap. For the studied ferroelectrics also the tensor of the second harmonic generation has been calculated and examined for a wide energy range. The results show good agreement with experimental measurements.