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.
Exact quantum dynamics calculations are performed for the bound rovibrational states of the neon tetramer (Ne4) in its ground electronic state, using pair-wise Lennard-Jones potentials and the ScalIT suite of parallel codes. The vibrational states separate into a low-lying group mostly localized to a single potential well and a higher-energy delocalized group lying above the isomerization threshold—with such a structure serving as a testament to the “intermediate” quantum nature of the Ne4 system. To accurately and efficiently represent both groups of states, the phase-space optimized discrete variable representation (PSO-DVR) approach was used, as implemented in the ScalIT code. The resultant 1D PSO effective potentials also shed significant light on the quantum dynamics of the system. All vibrational states were computed well up into the isomerization band and labeled up to the classical isomerization threshold—defined as the addition of the classical energy of a single bond, ϵ = 24.7 cm−1, to the quantum ground state energy. Rovibrational energy levels for all total angular momentum values in the range J = 1–5 were also computed, treating all Coriolis coupling exactly.
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. A 2006, 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.
This paper presents a practical guide for use of the ScalIT software package to perform highly accurate bound rovibrational spectroscopy calculations for triatomic molecules. At its core, ScalIT serves as a massively scalable iterative sparse matrix solver, while assisting modules serve to create rovibrational Hamiltonian matrices, and analyze computed energy levels (eigenvalues) and wavefunctions (eigenvectors). Some of the methods incorporated into the package include: phase space optimized discrete variable representation, preconditioned inexact spectral transform, and optimal separable basis preconditioning. ScalIT has previously been implemented successfully for a wide range of chemical applications, allowing even the most state-of-the-art calculations to be computed with relative ease, across a large number of computational cores, in a short amount of time.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.