The multi-reference correlation consistent composite approach (MR-ccCA) was designed to reproduce the accuracy of more computationally intensive ab initio quantum mechanical methods like MR-ACPF-DK/aug-cc-pCV?Z-DK, albeit at a significantly reduced cost. In this dissertation, the development and applications of the MR-ccCA method and a variant of its single reference equivalent (the relativistic pseudopotential ccCA method) are reported. MR-ccCA is shown to predict the energetic properties of reactive intermediates, excited states species and transition states to within chemical accuracy (i.e. ±1.0 kcal mol 1) of reliable experimental values. The accuracy and versatility of MR-ccCA are also demonstrated in the prediction of the thermochemical and spectroscopic properties (such as atomization energies, enthalpies of formation and adiabatic transition energies of spin-forbidden excited states) of a series of silicon-containing compounds. The thermodynamic and kinetic feasibilities of the oxidative addition of an archetypal arylglycerol ?-aryl ether (?-O-4 linkage) substructure of lignin to Ni, Cu, Pd and Pt transition metal atoms using the efficient relativistic pseudopotential correlation consistent composite approach within an ONIOM framework (rp-ccCA-ONIOM), a multi-level multi-layer QM/QM method formulated to enhance the quantitative predictions of the chemical properties of heavy element-containing systems larger than hitherto attainable, are also reported.

The aim of this thesis is to investigate strength and origin of the stabilization for various types of noncovalent interactions. As this knowledge could lead to a deeper understand- ing and rationalization of the binding phenomena. Further, to participate on the de- velopment of new noncovalent data sets, which are nowadays inevitable in the process of parametrization and validation of new computational methods. In all the studies, different binding motifs of model complexes, which represent usually crystal structures, structures from unrelaxed scans or the local minima, were investi- gated. The calculations of the reference stabilization energies were carried out at ab initio level (e.g. CCSD(T)/CBS, QCISD(T)/CBS). Further, the accuracy of more ap- proximate methods (e.g. MP2.5, DFT-D or SQM methods) toward reference method, was tested. In order to obtain the nature of the stabilization the DFT-SAPT decompo- sition was frequently utilized. In the first part of the thesis, the importance and basic characteristics of different types of noncovalent interactions (e.g. halogen bond, hydrogen bond, π· · · π interaction etc.), are discussed. The second part provides the description of computational methods which were essential for our investigation. The third part of the thesis provides an overview for part...