Smaller cellular telephones and more energy-efficient windows are just two examples of technological advances which call for new materials. Materials chemists seek to develop new materials, both out of pure curiosity to see which combination of elements and structures can be obtained and in efforts to produce materials, with specific properties. The starting materials (in solid, liquid or gaseous form) can then be combined and prepared in various ways. A chemical method that is gaining more attention for thin-film growth is Atomic Layer Deposition (ALD). This is a sophisticated type of vapor deposition in which the precursor gases are introduced separately into the reaction chamber. Silicon carbide (SiC) and cubic boron nitride (c-BN) are extremely hard diamond-like materials, both with a high potential for application within the modern microelectronics and tool industry. Hexagonal boron nitride (h-BN), with its graphite-like layered structure, is a promising ceramics material. Deposition of thin SiC and BN films from gaseous precursors has been studied by theoretical and experimental methods. The chemical composition and atomic arrangement of a growing surface is important for vapor growth. The surface may be terminated (e.g., by hydrogen atoms) and adopt various geometrical structures. Reconstruction of unterminated SiC(0001) surfaces, as well as H abstraction from the corresponding H-terminated surfaces, were studied using quantum mechanical calculations. Elementary reactions for vapor growth of SiC and BN, and in situ incorporation of dopant and contaminant species into these surfaces were also investigated theoretically. Moreover, thin films of BN were deposited by means of laser-assisted ALD. The general goal has been to predict and/or explain experimental results by investigating growth mechanisms.

Possibly the most fascinating biochemical mechanism remaining to be solved is the formation of oxygen from water in photosystem II. This is a critical part of the photosynthetic reaction that makes solar energy accessible to living organisms. The present thesis uses quantum chemistry, more specifically the density functional B3LYP, to investigate a mechanism where an oxyl radical bound to manganese is the active species in O-O bond formation. Benchmark calculations on manganese systems confirm that B3LYP can be expected to give accurate results. The effect of the self-interaction error is shown to be limited. Studies of synthetic manganese complexes support the idea of a radical mechanism. A manganese complex with an oxyl radical is active in oxygen formation while manganese-oxo complexes remain inactive. Formation of the O-O bond requires a spin transition but there should be no effect on the rate. Spin transitions are also required in many short-range electron-transfer reactions. Investigations of the superproficient enzyme orotidine decarboxylase support a mechanism that involves an invariant network of charged amino acids, acting together with at least two mobile water molecules.

Oscillator strengths and transition energies are calculated for several mono-substitutes of benzene and naphthalene molecules. The substituents investigated are chlorine, bromine and iodine. Calculations for these molecules are presented, at the Hartree-Fock and DFT level of theory. The functional used in DFT is CAM-B3LYP.

Despite the success of modern quantum chemistry in predicting properties of organic molecules, the treatment of inorganic systems, which have many close lying states, remains out of quantitative reach for current methods. To treat non-dynamic correlation, we take advantage of the density matrix renormalization group (DMRG) method that has become very successful in the field of solid state physics. We present a detailed study of the DMRG method, and we pay special attention to the evolution of the understanding behind the mathematical structure of the DMRG wave function. Our primary goal is to develop a density matrix renormalization group self--consistent--field (DMRG-SCF) approach, analogous to the complete active space self--consistent field (CASSCF) method, but dealing with large active spaces that are too demanding for the full configuration interaction (FCI) method. As a first step towards such a DMRG-SCF procedure, we present a spin-adapted DMRG algorithm designed to target spin- and spatial-symmetry states that are hard to obtain while using an unrestricted algorithm. Our next step is a modification of the DMRG algorithm to obtain decreasing energy at every step during the sweep. This monotonically convergent DMRG scheme lets us obtain the two-body density matrix as a by--product of the existing procedure without any additional cost in storage. Additionally, the two-body density matrix produced at convergence using this scheme is free from the N-representability problem that is present when the two--body density matrix is produced with the two-site DMRG scheme without additional storage cost. Finally, taking advantage of the modifications developed herein, we present results obtained from our DMRG-SCF method. Lastly, we discuss possible ways of incorporating dynamical correlation into the DMRG scheme, in order to obtain a modern multireference approach.

Developing chemical bonding models in clusters is one of the most challenging tasks of modern theoretical chemistry. There are two reasons for this. The first one is that clusters are relatively new objects in chemistry and have been extensively studied since the middle of the 20th century. The second reason is that clusters require high-level quantum-chemical calculations; while for many classical molecules their geometry and properties can be reasonably predicted by simpler methods. The aim of this dissertation was to study doped clusters and explain their chemical bonding. The research was focused on three classes of compounds: aluminum clusters doped with one nitrogen atom, planar compounds with hypercoordinate central atom, partially mixed carbon-boron clusters, and transition metal clusters. The geometry of the two latter classes of compounds was explained using the concept of aromaticity, previously developed in our group. Also the Coalescence Kick Method for finding global minima structure and low-lying isomers was implemented, tested, and applied to the considered cluster systems. Tests showed that the Kick Method works faster than other methods and provides reliable results. It finds global minima even for such large clusters as B17- and B19- in reasonable time.

Despite the success of modern quantum chemistry in predicting properties of organic molecules, the treatment of inorganic systems, which have many close lying states, remains out of quantitative reach for current methods. To treat non-dynamic correlation, we take advantage of the density matrix renormalization group (DMRG) method that has become very successful in the field of solid state physics. We present a detailed study of the DMRG method, and we pay special attention to the evolution of the understanding behind the mathematical structure of the DMRG wave function. Our primary goal is to develop a density matrix renormalization group self--consistent--field (DMRG-SCF) approach, analogous to the complete active space self--consistent field (CASSCF) method, but dealing with large active spaces that are too demanding for the full configuration interaction (FCI) method. As a first step towards such a DMRG-SCF procedure, we present a spin-adapted DMRG algorithm designed to target spin- and spatial-symmetry states that are hard to obtain while using an unrestricted algorithm. Our next step is a modification of the DMRG algorithm to obtain decreasing energy at every step during the sweep. This monotonically convergent DMRG scheme lets us obtain the two-body density matrix as a by--product of the existing procedure without any additional cost in storage. Additionally, the two-body density matrix produced at convergence using this scheme is free from the N-representability problem that is present when the two--body density matrix is produced with the two-site DMRG scheme without additional storage cost. Finally, taking advantage of the modifications developed herein, we present results obtained from our DMRG-SCF method. Lastly, we discuss possible ways of incorporating dynamical correlation into the DMRG scheme, in order to obtain a modern multireference approach.

The past decades have witnessed the development of new materials with large nonlinear optical properties, which have made them attractive candidats for a broad spectrum of breakthrough applications in the electro-optic and photonic fields (e.g., telecommunication and computing). A deeper understanding of the relationship between, on the one hand, the chemical structure and, on the other hand, the electronic and (linear and nonlinear) optical properties has proven useful for the rational design of new efficient materials. Reaching such an understanding has attracted major interest in the scientific community worldwide in both academia and industry. Therefore, the development of new efficient NLO chromophores and materials along with commercial devices of high quality is helped via the establishment of multidisciplinary research teams combining: (i) the theoretical modeling using quantum-chemical computational calculations; (ii) the organic synthesis; (iii) the optical characterization; and (iv) the device fabrication. In this dissertation, quantum-chemistry is used to evaluate the second- and third-order NLO properties of series of new chromophores and take advantage of a feedback loop with the experimental team to understand the structure-property relationships.

Nitrous oxide decomposition on Ag51, Au51, Pt22, Rh51 and Ir51 clusters representing (111) surface were studied quantum mechanically by using the method of ONIOM with high layer DFT region and low layer of molecular mechanics region utilizing universal force field (UFF). The basis set employed in the DFT calculations is the Los Alamos LANL2DZ effective core pseudo-potentials (ECP) for silver, gold, platinum, rhodium and iridium and 3-21G** for nitrogen, oxygen and hydrogen. Nitrous oxide was decomposed on the all metal surfaces investigated in this study by leaving oxygen atom adsorbed as supported by experimental findings. Activation energies of nitrous oxide decomposition on Ag51, Au51, Pt22, Rh51 and Ir51 representing (111) surface are calculated as 14.48 kcal/mol, 15.72 kcal/mol, 7.02 kcal/mol, 3.76 kcal/mol and 5.51 kcal/mol, respectively. Based on these results, decomposition of nitrous oxide occurs on Rh more easily than other metals.

In this work pentacene adsorption on both flat and stepped Ag(111) surfaces were investigated by using Density Functional Theory within Projected Augmented Wave method. On the flat Ag(111) surface favorable adsorption site for a single pentacene molecule was determined to be the bridge site with an angle of 60&amp / #9702 / between pentacene molecular long axis and [011] lattice direction. Potential energy surface was found to be flat, especially along lattice directions. Diffusion and rotation barriers for pentacene on this surface were found to be smaller than 40 meV indicating the possibility of a two dimensional gas phase. Calculated adsorption energies for the flat surface indicate a weak interaction between molecule and the surface indicating physisorption. On the flat surface monolayer case is found to have lower adsorption energy than the isolated case due to pentacene&amp / #8722 / pentacene interactions. On the stepped Ag(233) surface, close to the step edge, adsorption energy increased significantly due to the stronger interaction between pentacene molecule and low coordinated silver step atoms. On the terraces of this surface, far from step edges, however a flat potential energy surface was observed similar to the case of flat Ag(111) surface. On the stepped surface pentacene found its favorable configuration as parallel to the step with a tilt angle similar to the observed thin film phase of pentacene on Ag(111) surface. Pentacene molecule showed small distortions on stepped surface and are closer to the silver step atoms 1 &Aring / more than the case of flat surface, hinting a chemical interaction as well as van der Waals interactions. However on Ag(799) surface, the perpendicular orientation of the pentacene molecule to the step direction showed no strong interaction due to less matching of carbon atoms with silver step atoms.

Ethylene adsorption was studied by use of DFT/B3LYP with basis set 6-31G(d,p) in Gaussian&lsquo / 03 software. It was found that ethylene adsorbs molecularly on the Ni13 nanocluster with &amp / #960 / adsorption mode. &amp / #960 / adsorption mode is studied for the Ni10 (1 1 1), Ni13 (1 0 0) and Ni10 (1 1 0) surface cluster as well. Relative energy values were calculated as &amp / #8722 / 50.86 kcal/mol, &amp / #8722 / 20.48 kcal/mol, &amp / #8722 / 32.44 kcal/mol and &amp / #8722 / 39.27 kcal/mol for Ni13 nanocluster, Ni10 (1 1 1), Ni13 (1 0 0) and Ni10 (1 1 0) surface cluster models, respectively. Ethylene adsorption energy was found inversely proportional to Ni coordination number when Ni10 (1 1 1), Ni13 (1 0 0) and Ni10 (1 1 0) cluster models and Ni13 nanocluster were compared with each other. DFT/B3LYP and basis set of 86-411(41d)G in Gaussian&lsquo / 03 was used to investigate Ni55 nanocluster. Ethylene adsorption on Ni55 nanocluster was studied by means of equilibrium geometry calculations with &amp / #960 / adsorption modes for two different coordination numbers as 6 and 8. The related adsorption energies were approximately found as -22.07 and -14.82 kcal/mol for these coordination numbers of surfaces, respectively. In addition, the binding energies stated in literature that are for Ni2 dimer and Ni13 nanoclusters were considered together with our binding energy results for Ni55 nanocluster. Accordingly, when a correlation line was drawn and the intercept of binding energies was obtained against the value of &amp / #8213 / n&amp / #8722 / 1/3&amp / #8214 / where n is the number of atoms in the cluster / the result of interception gives a good estimation for bulk nickel binding energy at infinite &amp / #8213 / n&amp / #8214 / . This interception result was found as 4.58 eV/atom where the experimental value is reported as 4.45 eV/atom for bulk in the literature. Ehtylene hydrogenation mechanisms were also investigated in terms of the resultant geometries and total energy required for the related mechanism steps.