Computational chemistry lies at the intersection of chemistry, physics, mathematics, and computer science, and can be used to explain the behavior of atoms and molecules, as well as to augment experiment. In this work, computational chemistry methods are used to predict structural and energetic properties of small molecules, i.e. molecules with less than 60 atoms. Different aspects of computational chemistry are examined in this work. The importance of examining the converged orbitals obtained in an electronic structure calculation is explained. The ability to more completely describe the orbital space through the extrapolation of energies obtained at increasing quality of basis set is investigated with the use of the Sapporo-nZP-2012 family of basis set. The correlation consistent Composite Approach (ccCA) is utilized to compute the enthalpies of formation of a set of molecules and the accuracy is compared with the target method, CCSD(T,FC1)/aug-cc-pCV∞Z-DK. Both methodologies are able to produce computed enthalpies of formation that are typically within 1 kcal mol-1 of reliable experiment. This demonstrates that ccCA can be used instead of much more computationally intensive methods (in terms of memory, processors, and time required for a calculation) with the expectation of similar accuracy yet at a reduced computational cost. The enthalpies of formation for systems containing s-block elements have been computed using the multireference variant of ccCA (MR-ccCA), which is designed specifically for systems that require an explicit treatment of nondynamical correlation. Density functional theory (DFT) has been used for the prediction of the structural properties of a set of lanthanide trihalide molecules as well as the reaction energetics for the rearrangement of diphosphine ligands around a triosmium cluster.

The aim of this thesis is to investigate strength and origin of the stabilization for various types of noncovalent interactions. As this knowledge could lead to a deeper understand- ing and rationalization of the binding phenomena. Further, to participate on the de- velopment of new noncovalent data sets, which are nowadays inevitable in the process of parametrization and validation of new computational methods. In all the studies, different binding motifs of model complexes, which represent usually crystal structures, structures from unrelaxed scans or the local minima, were investi- gated. The calculations of the reference stabilization energies were carried out at ab initio level (e.g. CCSD(T)/CBS, QCISD(T)/CBS). Further, the accuracy of more ap- proximate methods (e.g. MP2.5, DFT-D or SQM methods) toward reference method, was tested. In order to obtain the nature of the stabilization the DFT-SAPT decompo- sition was frequently utilized. In the first part of the thesis, the importance and basic characteristics of different types of noncovalent interactions (e.g. halogen bond, hydrogen bond, π· · · π interaction etc.), are discussed. The second part provides the description of computational methods which were essential for our investigation. The third part of the thesis provides an overview for part...

Research Doctorate - Doctor of Philosophy (PhD) / (**Note: this abstract is a plain text version of the author's abstract, the original of which contains characters and symbols which cannot be accurately represented in this format. The properly formatted abstract can be viewed in the Abstract and Thesis files above.**) The electronic structure and rovibrational spectroscopy of MH2, MHn+2, HMHen+ and MHen+2 (M = Li, Be, Na, Mg, K, Ca; n = 1, 2) have been investigated using correlated ab initio ansatz. In order to determine the efficacy of various electronic structure methods with respect to Group-I and II hydrides and helides, atomic properties of Li, Be,Na, Mg, K and Ca were calculated. Relativistically-corrected UCCSD(T) and ICMRCI(+Q) were deemed to be the most suitable ansatz with respect to both efficiency and accuracy. The lowest 2A1 and 2Σ- states of MH2 were found to be purely repulsive, in agreement with previous predictions. The main factor determining the structure and stability of the excited states of MH2 was the relative orientations and occupations of the valence p atomic orbital of M and the H2 1Ou orbital. The ground states of MHn+2 were found to be the result of the charge-quadrupole interaction between Mn+ and the H2 molecular subunit. The structures of the ground states of HMHe+ were extremely uxional with respect to the central bond angle co-ordinate. The ground state PESs of MHe+2 were also extremely sensitive to the ab initio ansatz by which they are modelled. The respective bonding of the H and He in both HMHe+ and HMHe2+ appeared to be charge-dependent in the case of Be, Mg and Ca. Despite the weak bonding observed for the Group-II hydrohelide and helide monocations, the corresponding dications each exhibit thermodynamically stable equilibria. The solution algorithm of von Nagy-Felsobuki and co-workers was employed in the calculation of vibrational and rovibrational spectra. This algorithm employed an Eckart-Watson Hamiltonian in conjunction with rectilinear normal co-ordinates. Vibrational and rovibrational Hamiltonian matrices were diagonalised using variational methods. This algorithm was extended so that the vibration transition moment integrals, and hence vibrational radiative properties, of linear triatomic molecules could be calculated. A method by which vibration-averaged structures are calculated was also developed and implemented. Analytical potential energy functions (PEFs) and dipole moment functions (DMFs) of (1A1)LiH+2, (1A1)NaH+2, (1A1)BeH2+2,(1A1)MgH2+2, (1Σ+g )BeHe2+2, (2Σ+)HBeHe2+, (1Σ+g )MgHe2+2 and (2Σ+)HMgHe2+ were developed using leastsquare regression techniques in conjunction with discrete ab initio grids. Vibrational structures and spectra of these species were subsequently calculated. In addition, the rovibrational spectra of (1A1)LiH+2, (1A1)NaH+2, (1A1)BeH2+2 and (1A1)MgH2+2 were calculated. For (1A1)LiH+2 and (1A1)LiD+2 , calculated rovibrational transition frequencies for J ≤ 10 and 0 ≤ K ≤ 3 were within ca. 0.1-0.2% of experimental values.

Research Doctorate - Doctor of Philosophy (PhD) / (**Note: this abstract is a plain text version of the author's abstract, the original of which contains characters and symbols which cannot be accurately represented in this format. The properly formatted abstract can be viewed in the Abstract and Thesis files above.**) The electronic structure and rovibrational spectroscopy of MH2, MHn+2, HMHen+ and MHen+2 (M = Li, Be, Na, Mg, K, Ca; n = 1, 2) have been investigated using correlated ab initio ansatz. In order to determine the efficacy of various electronic structure methods with respect to Group-I and II hydrides and helides, atomic properties of Li, Be,Na, Mg, K and Ca were calculated. Relativistically-corrected UCCSD(T) and ICMRCI(+Q) were deemed to be the most suitable ansatz with respect to both efficiency and accuracy. The lowest 2A1 and 2Σ- states of MH2 were found to be purely repulsive, in agreement with previous predictions. The main factor determining the structure and stability of the excited states of MH2 was the relative orientations and occupations of the valence p atomic orbital of M and the H2 1Ou orbital. The ground states of MHn+2 were found to be the result of the charge-quadrupole interaction between Mn+ and the H2 molecular subunit. The structures of the ground states of HMHe+ were extremely uxional with respect to the central bond angle co-ordinate. The ground state PESs of MHe+2 were also extremely sensitive to the ab initio ansatz by which they are modelled. The respective bonding of the H and He in both HMHe+ and HMHe2+ appeared to be charge-dependent in the case of Be, Mg and Ca. Despite the weak bonding observed for the Group-II hydrohelide and helide monocations, the corresponding dications each exhibit thermodynamically stable equilibria. The solution algorithm of von Nagy-Felsobuki and co-workers was employed in the calculation of vibrational and rovibrational spectra. This algorithm employed an Eckart-Watson Hamiltonian in conjunction with rectilinear normal co-ordinates. Vibrational and rovibrational Hamiltonian matrices were diagonalised using variational methods. This algorithm was extended so that the vibration transition moment integrals, and hence vibrational radiative properties, of linear triatomic molecules could be calculated. A method by which vibration-averaged structures are calculated was also developed and implemented. Analytical potential energy functions (PEFs) and dipole moment functions (DMFs) of (1A1)LiH+2, (1A1)NaH+2, (1A1)BeH2+2,(1A1)MgH2+2, (1Σ+g )BeHe2+2, (2Σ+)HBeHe2+, (1Σ+g )MgHe2+2 and (2Σ+)HMgHe2+ were developed using leastsquare regression techniques in conjunction with discrete ab initio grids. Vibrational structures and spectra of these species were subsequently calculated. In addition, the rovibrational spectra of (1A1)LiH+2, (1A1)NaH+2, (1A1)BeH2+2 and (1A1)MgH2+2 were calculated. For (1A1)LiH+2 and (1A1)LiD+2 , calculated rovibrational transition frequencies for J ≤ 10 and 0 ≤ K ≤ 3 were within ca. 0.1-0.2% of experimental values.