Ab initio quantum chemical predictions of structures, energetics and physical properties: from environmentally active molecules to small clusters to dna base pairs.

Ab initio quantum chemistry (AIQC) methods have been employed in order to investigate small environmentally active triatomic NO2 to large DNA base pairs in this thesis. The reliability of predicting structures, energetics and physical properties of molecular systems are discussed with an eye on the applicability of density functional theory and highly correlatedmethods. Structures and spectroscopic parameters are predicted for several excited electronic states of NO2. An unbiased stochastic method to search for minima on the potential energy surfaces of molecules or clusters is developed and applied to variety of molecules. Biologicallyimportant topic of DNA damage by radiation is also addressed by predicting the gas phase radicals and ions and their molecular properties.