Density Functional Theory (DFT) is the most widely used theoretical tool for studying actinide complexes amongst the many available computational methods. However, the best choice of functional for theoretical actinide science is still not completely clear. Thus, benchmarking studies have been performed in this thesis in order to evaluate the performance of modern DFT applied to actinide systems. The first chapter is an introductory chapter which gives the background and the methods applied in this thesis. The second chapter is a part of the actinide spectroscopy Round-Robin test (RRT). The performance of DFT using different quantum chemistry programs with identical DFT functionals has been investigated by studying five U(VI) acetate complexes. The experimental data from the other clusters of RRT are used as reference for the quantum chemical calculations. The performance of a total of 22 different DFT functionals for small uranium complexes has been further investigated in Chapter 3. These functionals are compared by calculating geometries, vibrations frequencies, and reaction enthalpies against experimental data and high level ab initio CCSD(T) calculations. The last chapter presents a summary of the works in this thesis as well as directions for future studies. / May 2016