Correlation consistent basis sets of double-ζ through quintuple-ζ quality for the alkali and alkaline earth metals Li, Be, Na, and Mg have been developed, including the valence (cc-pVnZ), augmented valence (aug-cc-pVnZ), core-valence (cc-pCVnZ), and weighted core-valence (cc-pwCVnZ) basis sets. The basis sets are also re-contracted for Douglas–Kroll scalar relativistic calculations and are found to be superior to non-relativistic basis sets in recovering scalar relativistic effects. CCSD(T) computations have been performed with these basis sets, and a series of properties have been examined, including atomic ionization potentials and electron affinities, optimized molecular geometries, harmonic vibrational frequencies, atomization energies, and enthalpies of formation for the molecules Li2, LiF, BeO, BeF, BeH2, BeF2, Na2, NaF, MgO, MgF, MgH2, and MgF2.

The development of the correlation consistent basis sets, cc-pVnZ (where n = D, T, Q, etc.) have allowed for the systematic elucidation of the intrinsic accuracy of ab initio quantum chemical methods. In density functional theory (DFT), where the cc-pVnZ basis sets are not necessarily optimal in their current form, the elucidation of the intrinsic accuracy of DFT methods cannot always be accomplished. This dissertation outlines investigations into the basis set requirements for DFT and how the intrinsic accuracy of DFT methods may be determined with a prescription involving recontraction of the cc-pVnZ basis sets for specific density functionals. Next, the development and benchmarks of a set of cc-pVnZ basis sets designed for the s-block atoms lithium, beryllium, sodium, and magnesium are presented. Computed atomic and molecular properties agree well with reliable experimental data, demonstrating the accuracy of these new s-block basis sets. In addition to the development of cc-pVnZ basis sets, the development of a new, efficient formulism of the correlation consistent Composite Approach (ccCA) using the resolution of the identity (RI) approximation is employed. The new formulism, denoted 'RI-ccCA,' has marked efficiency in terms of computational time and storage, compared with the ccCA formulism, without the introduction of significant error. Finally, this dissertation reports three separate investigations of the properties of FOOF-like, germanium arsenide, and silicon hydride/halide molecules using high accuracy ab initio methods and the cc-pVnZ basis sets.

A set of 24 mixed silicon hydrides and halides (SiHnXm−n, where X=F, Cl; m=1–4; and n=0–m) has been investigated using; (1) CCSD(T) with the correlation consistent basis sets of double-through quintuple-ζ quality; (2) CCSD(T) with the augmented tight d form of the correlation consistent basis sets for the second row atoms (Si and Cl); and (3) the recently developed correlation consistent composite approach (ccCA). Optimized geometries and harmonic vibrational frequencies are reported, as well as enthalpies of formation, free energies of formation, and dissociation reaction enthalpies. Relativistic effects, including atomic spin-orbit coupling and scalar relativistic effects, and core-valence correlation, have been accounted for via the ccCA formulism. As demonstrated in previous studies, the tight d basis sets accelerate the convergence, as compared to the standard correlation consistent basis sets, of CCSD(T)-computed geometries and energetics of molecular species containing second row, main group atoms. The ccCA enthalpies of formation are shown to be similar and, on average, better than the corresponding CCSD(T) enthalpies of formation for the silicon species. Further, large DKH-CCSD(T) computations with explicit inclusion of core-valence and scalar relativistic effects have been performed to predict the enthalpies of formation of SiHF, SiHCl, SiH2F, SiHF2, SiH2Cl, and SiHCl2.

A new implementation of the correlation consistent composite approach (ccCA), denoted RI-ccCA, utilizing both the resolution of the identity (RI) and local methods is presented. A set of 102 molecules composed of first and second row, main group atoms is employed to compare total energies, atomization energies, and enthalpies of formation between the original ccCA implementation and those of RI-ccCA. Relative CPU time and disk space requirements of RI-ccCA as compared to ccCA, demonstrate that on the average, employing the RI approximation in ccCA affords CPU time savings over 70% and disk space requirements diminished by well over 90% without introducing significant error in energetic properties.

The ground state structures of the linear L–Ge–As and bent Ge–As–L systems (where L = H, Li, Na, BeH, MgH, BH2, AlH2, CH3, SiH3, NH2, PH2, OH, SH, F, Cl, and Br) have been investigated employing coupled cluster and density functional theory with the correlation consistent basis sets of double- through quadruple-ζ quality. The barriers to migration of the L group from the germanium terminus to the arsenic terminus have been determined by examining the transition state structure connecting the L–Ge–As and Ge–As–L minimum energy structures. Based on the computed transition states and classical barrier heights, new assignments in the relative stability of L–Ge–As compared with Ge–As–L, with respect to the L group, are proposed. The thermodynamic stability of these compounds is considered based on computed enthalpies of formation and enthalpies of isomerization. Relativistic effects are examined using two different approaches: the spin-free Cowan–Griffin and spin-free Douglas–Kroll–Hess Hamiltonians. Their impact on the atomization energies, enthalpies of formation, and enthalpies of isomerization upon germanium arsenide species is examined.

The interaction of benzene with a Ag(111) surface has been determined using reliable ab initio electronic structure calculations. The results are compared to a recent detailed analysis of the interaction of benzene with copper and gold surfaces, thus making it possible to derive a consistent picture for the electronic structure changes encountered when benzene is brought into contact with the densely packed coinage metal surfaces. To avoid the problems encountered when the presently most frequently employed computational approach, density functional theory (DFT), is applied to adsorbate systems where dispersion (or van der Waals) forces contribute substantially, we use a wavefunction-based approach. In this approach, the weak van der Waals interactions, which are dominated by correlation effects, are described using second-order perturbation theory. The surface dipole moment and the work function changes induced upon adsorption are also discussed.

Systematic truncation of the correlation consistent basis sets has been investigated in first and second row atoms and molecules to better understand basis set requirements for density functional theory, particularly the need for high angular momentum functions, as well as to understand possible computational cost savings that could be achieved by using reduced basis sets. The truncation scheme employed follows that recently introduced for ab initio methods [B. Mintz et al., J. Chem. Phys. 121, 5629 (2004)]. Properties examined in the current study include geometries, ionization potentials, electron affinities, and dissociation energies. In general, this investigation shows that a degree of truncation of higher angular momentum functions is possible with limited impact upon energetic properties, and does result in useful CPU time savings. However, not all properties investigated have the same level of dependence upon high angular momentum functions, and, thus, careful selection of truncated basis sets should be made.

The structures of the dihalogen-μ-dichalcogenides FSSF, ClSSCl, BrSSBr, FSeSeF, ClSeSeCl, and BrSeSeBr have been studied using both density functional theory and coupled cluster theory. Other isomers with the empirical formula A2X2 are presented briefly, and it is demonstrated that the XAAX (gauche) isomer is the lowest energy structural form. Four families of correlation consistent basis sets of double- through quadruple-ζ quality were used, including the standard, augmented, tight d (core polarization), and augmented tight d sets. Fully optimized structures and vibrational frequencies, including anharmonic computations, are reported and compared with available experimental results. In the fluorine compounds, the A–A bond is shown to be shorter than its corresponding A2 specie, which also occurs for the FOOF molecule [R.H. Jackson, J. Chem. Soc., (1962) 4585]. As well, increasing the halogen size increases the length of the A–A bond. This investigation demonstrates that this unique geometry is a consequence of anomeric delocalization of a chalcogen lone pair into an adjacent σ∗ orbital.

The correlation consistent basis sets of Dunning and coworkers, cc-pVnZ (n = D, T, Q, and 5) have been recontracted for use in BLYP and B3LYP density functional computations. A set of first row atoms (H, B-Ne) and molecules has been used to investigate properties including total energies, ionization potentials, electron affinities (vertical attachment energies), atomization energies, and optimized geometries. The impact of basis set superposition error on these properties also has been examined. Compared with the original Hartree-Fock contractions, these Kohn-Sham contractions are found to be superior for energetics, but have little impact on geometries. Extrapolations to the complete basis set (Kohn-Sham) limit have also been performed for energetic properties using the newly recontracted sets, and have been compared with extrapolations using the original cc-pVnZ basis sets. We have found that recontracting the basis sets, correcting for basis set superposition error, and including diffuse functions in the basis set are all needed to enable monotonic convergence of energetic properties with respect to increasing basis set size for all of the molecules investigated in this study.