The cyclopropenylidenyl carbene, c-(C)C3H2, should make for an excellent probe of unidentified infrared bands. It has a dipole moment of roughly 5.0 D making it easily detectable rotationally from the ground. Furthermore, it has vibrational frequencies computed here with proven and high-level quantum chemical methods that line up rather well with the typical C−H stretch, C−C stretch, out-of-plane wag, etc., bins delineated for polycyclic aromatic hydrocarbon fundamental frequencies. For instance, the bright C = C stretches are predicted to be at 5.474 and 6.394 μm, in line with the aromatic infrared bands observed toward various astrophysical regions and within the range of the EXES instrument on board the Stratospheric Observatory for Infrared Astronomy. As a result, potential radioastronomical detection of this molecule could be followed with IR analysis leading to a rare two-pronged analysis for this hydrocarbon, which should shed light onto the nature of currently unattributed IR features.

The growth of commercial cloud computing (CCC) as a viable means of computational infrastructure is largely unexplored for the purposes of quantum chemistry. In this work, the PSI4 suite of computational chemistry programs is installed on five different types of Amazon World Services CCC platforms. The performance for a set of electronically excited state single-point energies is compared between these CCC platforms and typical, "in-house" physical machines. Further considerations are made for the number of cores or virtual CPUs (vCPUs, for the CCC platforms), but no considerations are made for full parallelization of the program (even though parallelization of the BLAS library is implemented), complete high-performance computing cluster utilization, or steal time. Even with this most pessimistic view of the computations, CCC resources are shown to be more cost effective for significant numbers of typical quantum chemistry computations. Large numbers of large computations are still best utilized by more traditional means, but smaller-scale research may be more effectively undertaken through CCC services.

Commercial cloud computing (CCC) has the promise of an untold number of computing nodes available for the researcher as long as he or she has the financial means to absorb these costs and the administrative skills necessary to effectively utilize the resources. The key is finding how to maximize parallelization for a minimum of monetary and management costs. Previous work has shown that CCC resources are viable for use on large numbers of small-to-medium sized quantum chemical computations. Composite energy quartic force fields (QFFs) are a highly-attractive platform for subsequent testing of CCC resources to find the proper balance between time savings of the cloud versus monetary expenditure. Use of this type of potential energy surface has lead to highly-accurate rovibrational data in earlier work. QFFs use large numbers of stand-alone energies that have to be computed for various molecular geometries. At each geometry, different methods and/or basis sets are used to efficiently generate accurate representations of the nuclear potential. For this initial study, the small molecular anion, SiCH 2 of interest in astrochemistry, is chosen for analysis as it can be done cheaply on the cloud while still providing insight into the nature of CCC usage. Additionally, no rovibrational data exists for this molecule making it the first molecule quantum chemically computed purely via CCC tools.

No abstract

The NS radical is a simple arrangement of atoms with a complex electronic structure. This molecule was first reported by Hassanzadeh and Andrew's group [J. Am. Chem. Soc. 114, 83 (1992)] through Ar matrix isolation experiments. In the quarter century since this seminal work was published, almost nothing has been reported about nitrogen disulfide even though NS is isovalent with the common NO. The present study aims to shed new insight into possible challenges with the characterization of this radical. No less than three potential energy surfaces all intersect in the C region of the SNS radical isomer. A type-C Renner-Teller molecule is present for the linear Π state where the potential energy surface is fully contained within the 2.05 kcal/mol lower energy X̃ A state. A C, 1 B state is present in this same region, but a double excitation is required to access this state from the X̃ A state of SNS. Additionally, a 1 A NSS isomer is also present but with notable differences in the geometry from the global minimum. Consequently, the rovibronic spectrum of these NS isomers is quite complicated. While the present theory and previous Ar matrix experiments agree well on isotopic shifts, they differ notably for the absolute fundamental vibrational frequency transitions. These differences are likely a combination of matrix shifts and issues associated with the neglect of non-adiabatic coupling in the computations. In either case, it is clear that high-resolution gas phase experimental observations will be complicated to sort. The present computations should aid in their analysis.