Over the past decades, the theoretical aspects of heterogeneous catalysis have grown in importance in this field with high industrial and societal implications. The present review is devoted to the increased complexity in the theoretical models employed in atomistic simulations. To this end, we describe the state-of-the-art studies of complex materials relevant for gas storage. After crystal complexity, the effect of the environment and impurities on the structural and chemical properties are presented. The third point discussed concerns the catalyst reactivity for the complex reaction networks. The analysis of reaction rates or selectivities directly comparable to experiments is shown in view of the information from individual reaction steps. Next, the origin of simplification rules is introduced. Finally, the challenges ahead of us are summarized showing possible future developments of theoretical simulations of materials with relevance in catalysis and energy conversion or storage.