Intro -- Abstract -- Popular science summary -- Populärvetenskaplig sammanfattning -- Preface -- Acknowledgements -- Contents -- Introduction -- Silicon and post-silicon information processing -- Essential requirements and possible implementations of quantum information processing -- Theoretical challenges in the defect spin implementation -- Aim of my research -- Theoretical approaches in solid state qubit studies -- Ab initio methods -- Basic equations and approximations -- Density functional theory -- The exchange and correlation functional -- Model spin Hamiltonians -- Spin Hamiltonian of important solid state qubits -- Group theory considerations -- Basic definitions -- Representations -- Group of the wave vector -- Character tables and product tables -- Hybrid-DFT+Vw scheme -- Analogy of generalized Kohn-Sham and quasi-particles -- Theoretical unification of hybrid-DFT and DFT+U methods for the treatment of localized orbitals -- Limitations of hybrid functionals in mixed systems -- Zero-field-splitting tensor calculation -- Implementation of zero-field-splitting tensor calculation -- Application in qubit characterization -- Temperature and pressure dependence -- Model of dynamic nuclear polarization -- Basic properties of electron and nuclear spin hybrid registers in diamond and SiC -- Simple model of dynamic nuclear polarization -- Comprehensive theoretical model of DNP -- DNP in SiC -- Ground state DNP of weakly coupled hybrid registers -- Group theory approaches -- Carbon antisite-vacancy pair -- Dirac points in novel 2D materials -- Conclusions and outlook -- The hybrid-DFT+Vw scheme -- Zero-field-splitting calculation -- Dynamic nuclear spin polarization -- Bibliography -- List of included Publications -- Related, not included publications -- Summary of included papers.