Thesis (M.Sc. (Physics)) --University of Limpopo, 2006 / In this work, we are studying the properties of CaS and CaO structures in both atomistic simulation and Density Functional Theory. Defects formation (vacancies, impurity and interstitial) will be mechanism studied by using atomistic simulation method. In this approach, Mott-Littleton method will be used since it is a good ap- proach of defects studies, and further explanation will be given on how the introduction of defects contribute on the stability of the bulk material. Diffusion of different atoms from one lattice site via interstitial path to vacancy lattice site, and how it segregates through the material, is also part of this study. The surface properties will be studied using both methods mentioned. Surface energies calculations of different surface layers (e.g. CaS (100), CaS (110), CaS (111), CaO (100), CaO (110) and CaO (111)) is the approach we used to determine the most stable surface. In atomistic simulation, we further studied how percentage coverage of atoms contributes on the stability of the surfaces. We further used Density Functional Theory to calculate surface energies of the above-mentioned surfaces. As in atomistic simulation method, we used surface ener- gies to determine the most stable surface. In DFT we used only the most stable surface of both CaS and CaO to study the adsorption of molecules, namely H2O, H2S, HS and S2 on CaO (100) and CaS (100). The most/least-adsorbed molecule on both surfaces is explained in this study.