The aim of the project is a comprehensive analysis of selected materials: a) chalcogenides with the stoichiometry AgXY2 (X: Sb, Sn, Pb, Y: Se, Te and xi, yi = 0 - 1) and b) silicon clathrates. Research on the impact of initial composition and local disorder in the host network (chalcogenides) and the type, amount of admixtures and positions occupied by them in the structure (chalcogenides and silicon clathrates) on thermoelectric properties of the materials studied will be based on ab initio calculations. Major part of the research will be devoted to the detailed description of the electronic structure of materials, i.e. density of states and band structure, but also topological analysis of the total electron density in a system within Bader's approach. Particular attention will be paid to local properties (related to atomic interactions), which have a very important impact on final thermoelectric properties of materials, often neglected in previous studies.