Application and development of quantum chemical methods. Density functional theory and valence bond theory

Abstract: This thesis deals with two disjoint subdiciplines of quantum chemistry.  One isthe most used electronic structure method today, density functional theory(DFT), and the other one of the least used electronic structure methods,valence bond theory (VB).  The work on DFT is based on previous developments inthe department in density functional response theory and involves studies ofhyperfine coupling constants which are measured in electron paramagneticresonance experiments.  The method employed is a combination of arestricted-unrestriced approaches which allows for adequate description of spinpolarization without spin contamination, and spin-orbit corrections to accountfor heavy atom effects useing degenerate perturbation theory.  The work anvalence bond theory is a new theoretical approach to higher-order derivatives.The orbital derivatives are complicated by the fact that the wave functions areconstructed from determinants of non-orthogonal orbitals. An approach based onnon-orthogonal second-quantization in biorthogonal basis sets leads tostraightforward derivations without explicit references to overlap matrices.These formulas are relevant for future applications in time-dependent valencebond theory.