| Combustion processes and polyaromatic hydrocarbon formation have been very active fields of research both experimentally and theoretically in the physics, chemistry, chemical engineering and mechanical engineering communities. Many of the molecular systems of interest in these fields are often difficult to study experimentally due to them being unstable, reactive radials with system sizes ranging from several to thousands of atoms. As a result, high accuracy quantum chemistry calculations are often needed to supplement experimental methods to comprehend these reactions. Quantum Monte Carlo methods, which have been shown in the past to be of high accuracy and to possess modest scaling of computational effort with systems size, have been applied to molecules that are of interest to combustion and polyaromatic hydrocarbon formation. Despite the frequent application of quantum Monte Carlo methods, there have been very few benchmark studies that gauge the accuracy of these methods on chemical systems, especially for hydrocarbons. Furthermore, zero-point energies and geometries are often taken from methods other than quantum Monte Carlo, since forces are still under active development. The lack of sufficient number of quantum Monte Carlo benchmark studies, in conjunction with the relatively high computational cost of the methods compared with other ab initio methods, reduces the popularity of quantum Monte Carlo methods despite their promise of high versatility and accuracy.;In this thesis, the accuracy of the diffusion Monte Carlo variant of the quantum Monte Carlo methods is examined for the computation of the thermodynamic quantities such as bond dissociation energy, atomization energies, and enthalpies of formation for 22 small hydrocarbon molecules. The isomers of C4H 3 and C4H5 are re-examined since published energy differences were substantially different than highly accurate ab initio methods such as coupled cluster.;The C-H bond dissociation energy of acetylene was found to be in substantial disagreement with experiment when calculated with a Hatree-Fock wave function in DMC. Thus, in an attempt to improve accuracy, acetylene and the ethynyl radical were studied with more sophisticated trial wave functions. It was found that using multi-determinant valence bond trial functions improved the diffusion Monte Carlo bond dissociation energies to match the experimental value. Furthermore, it is proposed that valence bond wave functions are preferable to truncated multi-configurational self-consistent field wave functions, when consistent correlation energy recovery is desired, such as for isomerization energies and bond dissociation energies. A proof of concept quantum Monte Carlo calculation with a valence bond trial wave function gives a reasonable inter-atomic force at the equilibrium geometry of F-2 .;Additionally, dihalogen anions are studied as prototypical 3-electron 2-center bonded test systems which are known to be problematic for density functional theory and perturbation theory methods due to artifactual symmetry breaking. Moller-Plesset perturbation theory with Kohn-Sham orbitals is found to give relatively good bond lengths, harmonic frequencies and dissociation energies for these systems. |
