Developments and applications of quantum chemistry : from novel electronic structure methods to conjugated optoelectronic materials

The work contained within this thesis encompasses a variety of different techniques to describe chemical systems of varying complexity, ranging from geometrically simple systems with an inherently correlated, and thus complex, electronic structure and vice versa. The stochastic technique of Monte-Carlo configuration interaction, which can generate properties of full configuration interaction (FCI) quality despite vast reduction in wavefunction size, was locally modified to involve a more systematic configuration selection regime, termed systematic-MCCI. A comparison of both approaches was undertaken on Ne, H2O, CO and Cr2 due to varying electronic structure in these systems; for comparison FCI and a novel pruned-FCI alternative was also included. It shall be demonstrated that the stochastic MCCI approach produced near optimal wavefunctions when compared to these systematically generated wavefunctions, with a far reduced computational cost. We then switch our attention to the modelling of chemical reaction pathways, specifically those undergoing an intramolecular Diels-Alder (IMD-A) cycloaddition process, using the high-accuracy thermochemistry method, CBS-QB3. It was observed that incorporation of a nitro group into the IMD-A substrate resulted in more exergonic reactions and lower activation barriers, when compared to non-substituted substrates, attributed to the enhanced positive charge stabilisation in the cycloadduct. The nature of the important frontier molecular orbital (FMO) interaction was also found to completely reverse upon nitration. The substitution of the dieneophile was tolerated to a much better degree in nitro bearing systems due to the increased distances between the bond forming carbons in the transition state. However, this nitration effect was not observed in highly polarity mismatched substrates. We also investigate if structureenergy correlations are present in a set of hetero IMD-A substrates, between the bond length contraction upon going from the transition state to the cycloadduct and the overall reaction free energy change and the retro IMD-A barrier. However, as will be discussed for the substrates of interest herein we observe very little correlation. In addition we explored the structure-energy correlation for the aforementioned nitro and non-nitrated IMD-A reactions but similar findings were observed. A density functional theory (DFT) study is then undertaken for a range of organic π-conjugated materials. We explore the ionisation potential of these materials with a comparison to recent experimental findings. As shall be discussed we observe good agreement with experiment, within 100 meV of the uncertainty range, for the computationally cheap approach of a single oligomer contained within an implicit solvent model when there is no known stacking within the polymer environment. However, if the thin film environment exhibits a degree of ordering this simple approach breaks down and more sophisticated models must be implemented. The thesis concludes with a molecular dynamics (MD) study of two simple π-organic conjugated materials using a recently parameterised force-field. The configurational landscape attained from the MD simulations are then explored with electronic structure methods and a brief statistical analysis undertaken