The pursuit of subchemical accuracy in quantum chemistry via basis set extrapolation techniques
Schuurman, Michael Stewart
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Techniques that employ the extrapolation of atomic basis sets, in concert with sophisticated ab initio treatments of electron correlation, ar e now capable of generating precise molecular properties and reaction energies of subchemical ( 0.1 kcal mol -1) accuracy. In this dissertation, these techniques ar e applied to a range of chemi cal systems. Computational thermochemistry is the focus of the first investig ation, in which the enthalpies of formation of NCO and the isomers of [H, N, C, O] are determined using formation reactions explicitly designed to minimize errors in the computed reac tion energies. The mean computed enthalpies of formation (from two indepe ndent NCO formation reactions and seven HNCO reactions) are given by 0 H f (NCO) = +30.5 ± 0.2, and 0 H f (HNCO) = –27.6 ± 0.2 kcal mol -1. To achieve such precision, additive corrections for core correlation, spin-orbit coup ling, special relativity, anharmonic zero-point vibrational energies, and non-Born-Oppenheimer effects, must be considered. In the second study, extrapolated gradient techniques are employed to obtain complete basis set (CBS) limit CCSD(T) geometries of BH 3 and BH 5, the structure of the latter species exhibiting a prodigious basis set dependence. Focal-point extrapolations yield a Cs-symmetry global minimum comprised of BH 3 and H2 subunits and featuring interfragment B – H distances of (1.401, 1.414) Å, an H2 bond length elongated to 0.803 Å, and a BH 3 + H2 dissociation energy De (D0) = 6.6 (1.2) kcal mol -1. The vibrationless barriers for H 2 internal rotation and hydrogen scrambling are 0.07 and 5.81 kcal mol -1, respectively. As a first step in investigating the extremely anharmonic 12-dimensi onal vibrational dynamics of BH 5, a complete quartic force field has been computed at the all-electron cc-pCVQZ CCSD(T) level of theory. The final application combines extrapolated gradient and energy techniques to construct a CBS limit CCSD(T) quartic force field for BH3. The reference structure (re = 1.18642 Å) for this force field was optimized employing extrapolated CCSD(T) gradients. The resulting vibrational frequencies, given by 1 = 2502.5 cm -1, 2 = 1147.4 cm -1, 3 = 2602.7 cm -1, and 4 = 1196.3 cm-1, display a mean absolu te error of only 0.5 cm -1 with infrared gas-phase fundamental frequencies.