Accurate rovibrational energies of ozone isotopologues up to J = 10 utilizing artificial neural networks
    In recent years, ozone and its isotopologues have been a topic of interest in many fields of research, due to its importance in atmospheric chemistry and its anomalous isotopic enrichment—or the so-called “mass-independent fractionation.” In the field of potential energy surface (PES) creation, debate over the existence of a potential barrier just under the dissociation threshold (referred to as a “potential reef”) has plagued research for some years. Recently, Dawes and co-workers [Dawes, Lolur, Li, Jiang, and Guo (DLLJG) J. Chem. Phys. 139, 201103 (2013)] created a highly accurate global PES, for which the reef is found to be replaced with a (monotonic) “plateau.” Subsequent dynamical calculations on this “DLLJG” PES have shown improved agreement with experiment, particularly the vibrational spectrum. However, it is well known that reaction dynamics is also highly influenced by the rovibrational states, especially in cases like ozone that assume a Lindemann-type mechanism. Accordingly, we present the first significant step toward a complete characterization of the rovibrational spectrum for various isotopologues of ozone, computed using the DLLJG PES together with the ScalIT suite of parallel codes. Additionally, artificial neural networks are used in an innovative fashion—not to construct the PES function per se but rather to greatly speed up its evaluation.
    Thermochemical and Kinetics of CH3SH + H Reactions: The Sensitivity of Coupling the Low and High-Level Methodologies
    The reaction system formed by the methanethiol molecule (CH3SH) and a hydrogen atom was studied via three elementary reactions, two hydrogen abstractions and the C–S bond cleavage (CH3SH + H → CH3S + H2 (R1); → CH2SH + H2 (R2); → CH3 + H2S (R3)). The stable structures were optimized with various methodologies of the density functional theory and the MP2 method. Two minimum energy paths for each elementary reaction were built using the BB1K and MP2 methodologies, and the electronic properties on the reactants, products, and saddle points were improved with coupled cluster theory with single, double, and connected triple excitations (CCSD(T)) calculations. The sensitivity of coupling the low and high-level methods to calculate the thermochemical and rate constants were analyzed. The thermal rate constants were obtained by means of the improved canonical variational theory (ICVT) and the tunneling corrections were included with the small curvature tunneling (SCT) approach. Our results are in agreement with the previous experimental measurements and the calculated branching ratio for R1:R2:R3 is equal to 0.96:0:0.04, with kR1 = 9.64 × 10–13 cm3 molecule^–1 s^–1 at 298 K.
    Dissertation: Exact Quantum Dynamical Calculations of Rovibrational Spectra Using Massively Parallel Computers
    We perform highly accurate rovibrational spectra calculations on the HO2 and Ne4 systems using ScalIT, an exact quantum dynamics software suite designed to perform such calculations across a massive number of computer processors in a straightforward manner. HO2 calculations are performed up to the dissociation threshold, corresponding to a total angular momentum value, J ≤ 130. A series of theory–based J–shifting (JS) schemes are also introduced and applied to a representative set of J values of HO2. The results are compared to both the previously mentioned exact values calculated, and experimentally derived vibrational-state-dependent JS results [J. Phys. Chem. A. 110, 3246, (2006)]. One of the introduced methods, the modified effective potential (modEP) scheme, outperforms all others in all regimes, and appears to be resistant to Coriolis-coupling effects. The modEP scheme is used as an analysis tool to shed structural insight on the dynamics of Coriolis-coupled eigenstate wave functions of HO2. The vibrational spectrum for Ne4 is calculated for all possible permutation inversion symmetries, and the physically real states are identified
    Quantum Dynamical Calculation of Bound Rovibrational States of HO2 up to Largest Possible Total Angular Momentum, J ≤ 130
    In a previous article [J. Theor. Comput. Chem.2010, 9, 435], all rovibrational bound states of HO2 were systematically computed, for all total angular momentum values J = 0–10. In this article, the high-J rovibrational states are computed for every multiple-of-ten J value up to J = 130, which is the point where the centrifugal barrier obliterates the potential well, and bound states no longer exist. The results are used to assess the importance of Coriolis coupling in this floppy system and to evaluate two different J-shifting schemes. Though not effective for multiply vibrationally excited bound states, vibrational-state-dependent J-shifting obtains modestly accurate predictions for the lowest-lying energies [J. Phys. Chem. A2006, 110, 3246]. However, much better performance is obtained—especially for large J values, and despite substantial Coriolis coupling—using a second, rotational-state-dependent J-shifting scheme [J. Chem. Phys.1998, 108, 5216], for which the rotational constants themselves depend on J and K. The latter formalism also yields important dynamical insight into the structure of the strongly Coriolis-coupled eigenstate wave functions. The calculations were performed using ScalIT, a suite of codes enabling quantum dynamics calculations on massively parallel computing architectures.