Theoretical developments and applications of electronic structure theory to problems in reaction dynamics and fundamental chemical concepts

In Section I, ab initio direct classical trajectory calculations have been carried out for studies of formaldehyde, glyoxal, and s-tetrazine photodissociations. The results are in very good agreement with experiments in various aspects, such as the rotational, vibrational and translational energy distributions of the product. The branching ratios for the dissociation of acetylene dication, C₂H₂ 2+, have been investigated at three different total available energies. The deprotonation channel was most abundant, while the two-step decabonation channel was not observed. The ADMP trajectories were compared for the dissociation of formaldehyde and glyoxal, and found in good agreement with the Born-Oppenheimer results. For the final product analysis in trajectory calculations, a simple uniform scaling approach was proposed to estimate the vibrational anharmonicity.; In some hydrogen bonding interactions, where X and Y are hydrogen donor and acceptor, respectively, the X-H stretching frequency is blue-shifted accompanied by bond contraction. Such unconventional blue-shifted hydrogen bonds were investigated in Section II. The physical origin was found to be an appropriate balance between attractive forces (orbital interaction, electrostatic interaction and dipole interaction) and repulsive forces (nuclei-nuclei repulsion and Pauli repulsion). Another weakly bonded system, π-anion interaction, has also been investigated. Such interactions were characterized in terms of geometric parameters, interaction strengths, and driving forces.; Density matrix search using direct inversion in the iterative subspace as a linear scaling alternative to diagonalization (QN-DMS) in electronic structure calculations was presented. Benchmarks of QN-DMS showed about 30% savings in computational cost compared to CG-DMS. A progressive geometry optimization method using the DIIS algorithm (P-DIIS) was introduced and showed about 25% computational savings for large molecule optimizations.; A time-dependent Hartree-Fock algorithm using atom-centered basis functions and the unitary transformation (UT-TDHF) was developed for studies of the electronic optical response. The UT-TDHF was shown to be a powerful and affordable non-perturbative approach to investigate electronic behavior of molecules in intense laser fields.