Mixed quantum/classical calculations of rotationally inelastic scattering in the CO + CO system: a comparison with fully quantum results.

An updated version of the CO + CO potential energy surface from [R. Dawes, X. G.

Rovibrational states calculations of the HO-HCN heterodimer with the multiconfiguration time dependent Hartree method.

Water and hydrogen cyanide are two of the most common species in space and the atmosphere with the ability of binding to form dimers such as HO-HCN. In the literature, while calculations characterizing various properties of the HO-HCN cluster (equilibrium distance, vibrational frequencies and rotational constants) have been done in the past, extensive calculations of the rovibrational states of this system using a reliable quantum dynamical approach have yet to be reported. In this work, we intend to mend that by performing the first calculation of the rovibrational states of the HO-HCN van der Waals complex on a recently developed potential energy surface.

Ab initio quantum scattering calculations and a new potential energy surface for the HCl(X1Σ+)-O2(X3Σg-) system: Collision-induced line shape parameters for O2-perturbed R(0) 0-0 line in H35Cl.

The remote sensing of abundance and properties of HCl-the main atmospheric reservoir of Cl atoms that directly participate in ozone depletion-is important for monitoring the partitioning of chlorine between "ozone-depleting" and "reservoir" species. Such remote studies require knowledge of the shapes of molecular resonances of HCl, which are perturbed by collisions with the molecules of the surrounding air. In this work, we report the first fully quantum calculations of collisional perturbations of the shape of a pure rotational line in H35Cl perturbed by an air-relevant molecule [as the first model system we choose the R(0) line in HCl perturbed by O2].

Temperature Dependence of the Electronic Absorption Spectrum of NO.

The nitrogen dioxide (NO) radical is composed of the two most abundant elements in the atmosphere, where it can be formed in a variety of ways including combustion, detonation of energetic materials, and lightning. Relevant also to smog and ozone cycles, together these processes span a wide range of temperatures. Remarkably, high-resolution NO electronic absorption spectra have only been reported in a narrow range below about 300 K.

Collisional excitation of C2H- by H2: New interaction potential and scattering calculations.

Interstellar anions play an important role in astrochemistry as being tracers of the physical and chemical conditions in cold molecular clouds and circumstellar gas. The local thermodynamic equilibrium is generally not fulfilled in media where anions are detected and radiative and collisional data are required to model the observed lines. The C2H- anion has not yet been detected in the interstellar medium; however, collisional data could be used for non-LTE models that would help in identifying the most intense lines.

Theoretical study of the CO-O van der Waals complex: potential energy surface and applications.

A four-dimensional-potential energy surface (4D-PES) of the atmospherically relevant carbon dioxide-oxygen molecule (CO-O) van der Waals complex is mapped using the explicitly correlated coupled cluster method with single, double, and perturbative triple excitations (UCCSD(T)-F12b), and extrapolation to the complete basis set (CBS) limit using the cc-pVTZ-F12/cc-pVQZ-F12 bases and the formula. An analytic representation of the 4D-PES was fitted using the method of interpolating moving least squares (IMLS). These calculations predict that the most stable configuration of CO-O complex corresponds to a planar slipped-parallel structure with a binding energy of ∼ -243 cm.

Ab initio quantum scattering calculations for the CO-O system and a new CO-O potential energy surface: O and air broadening of the R(0) line in CO.

We present ab initio calculations of the collisional broadening of the R(0) pure rotational line in CO (at 115 GHz) perturbed by O. Our calculations are done in a fully quantum way by solving close-coupling quantum-scattering equations without any approximations. We also report a new, highly accurate CO-O potential energy surface on which we did the quantum-scattering calculations.

Fully quantum calculations of O-N scattering using a new potential energy surface: Collisional perturbations of the oxygen 118 GHz fine structure line.

A proper description of the collisional perturbation of the shapes of molecular resonances is important for remote spectroscopic studies of the terrestrial atmosphere. Of particular relevance are the collisions between the O and N molecules-the two most abundant atmospheric species. In this work, we report a new highly accurate O(XΣ )-N(XΣ ) potential energy surface and use it for performing the first quantum scattering calculations addressing line shapes for this system.

The Low-Lying Electronic States of NO: Potential Energy and Dipole Surfaces, Bound States, and Electronic Absorption Spectrum.

Nitrogen dioxide, NO, is a free radical composed of the two most abundant elements in Earth's atmosphere, nitrogen and oxygen, and is relevant to atmospheric and combustion chemistry. The electronic structure of even its lowest-lying states is remarkably complex, with various conical intersections and Renner-Teller pairings, giving rise to complex and perturbed vibronic states. Here we report some analysis of the 18 molecular states of doublet spin-multiplicity formed by combining ground-state N(S) and O(P) atoms.

Inelastic scattering in isotopologues of O-Ar: the effects of mass, symmetry, and density of states.

The two species considered here, O2 (oxygen molecule) and Ar (argon-atom), are both abundant components of Earth's atmosphere and hence familiar collision partners in this medium. O2 is quite reactive and extensively involved in atmospheric chemistry, including Chapman's cycle of the formation and destruction of ozone; while Ar, like N2, typically plays the nevertheless crucial role of inert collider. Inert species can provide stabilization to metastable encounter-complexes through the energy transfer associated with inelastic collisions.

Spectroscopy and Scattering Studies Using Interpolated Ab Initio Potentials.

The Born-Oppenheimer potential energy surface (PES) has come a long way since its introduction in the 1920s, both conceptually and in predictive power for practical applications. Nevertheless, nearly 100 years later-despite astonishing advances in computational power-the state-of-the-art first-principles prediction of observables related to spectroscopy and scattering dynamics is surprisingly limited. For example, the water dimer, (HO), with only six nuclei and 20 electrons, still presents a formidable challenge for full-dimensional variational calculations of bound states and is considered out of reach for rigorous scattering calculations.

Collisional excitation of interstellar PN by H: New interaction potential and scattering calculations.

Rotational excitation of interstellar PN molecules induced by collisions with H is investigated. We present the first ab initio four-dimensional potential energy surface (PES) for the PN-H van der Waals system. The PES was obtained using an explicitly correlated coupled cluster approach with single, double, and perturbative triple excitations [CCSD(T)-F12b].

Computational study of the rovibrational spectrum of CO-N.

The CO2-N2 complex is formed from two key components of Earth's atmosphere, and as such, has received some attention from both experimental and theoretical studies. On the theory side, a potential energy surface (PES) based on high level ab initio data was reported [Nasri et al., J.

Automated Construction of Potential Energy Surfaces Suitable to Describe van der Waals Complexes with Highly Excited Nascent Molecules: The Rotational Spectra of Ar-CS() and Ar-SiS().

Some reactions produce extremely hot nascent products which nevertheless can form sufficiently long-lived van der Waals (vdW) complexes-with atoms or molecules from a bath gas-as to be observed via microwave spectroscopy. Theoretical calculations of such unbound resonance states can be much more challenging than ordinary bound-state calculations depending on the approach employed. One encounters not just the floppy, and perhaps multiwelled potential energy surface (PES) characteristic of vdWs complexes, but in addition, one must contend with excitation of the intramolecular modes and its corresponding influence on the PES.

Rotationally inelastic scattering of O-Ar: state-to-state rates with the multiconfigurational time dependent Hartree method.

The Chapman cycle, proposed in 1930, describes the various steps in the ongoing formation and destruction of stratospheric ozone. A key step in the formation process is the stabilization of metastable ozone molecules through collisions with a third body, usually an inert collider such as N. The "ozone isotopic anomaly" refers to the observation of larger-than-expected atmospheric concentrations for certain ozone isotopologues.

Computational study of the ro-vibrational spectrum of CO-CO.

An accurate ab initio ground-state intermolecular potential energy surface (PES) was determined for the CO-CO van der Waals dimer. The Lanczos algorithm was used to compute rovibrational energies on this PES. For both the C-in and O-in T-shaped isomers, the fundamental transition frequencies agree well with previous experimental results.

Collisional Excitation of CF by H: Potential Energy Surface and Rotational Cross Sections.

The CF molecule is considered one of the key species for the study of fluorine chemistry in the interstellar medium (ISM). Its recent detection, as well as its potential use as a tracer for atomic fluorine in the ISM, has increased the interest in the study of the physical and chemical properties of this cation. Accurate determination of the CF abundance in the ISM requires detailed modeling of its excitation from both radiation and collisions with the most dominant species, which are usually atomic and molecular hydrogen.

Development of a potential energy surface for the O-Ar system: rovibrational states of the complex.

The cycle of formation and destruction of ozone is an important process in the atmosphere. A key step in the formation process is the stabilization of a metastable ozone molecule, which occurs through energy transfer: usually a highly excited ozone molecule loses the excess energy through inelastic collisions with a third body (M). However, the details of this energy transfer mechanism are still not well known and one of the reasons has been the lack of an accurate potential energy surface (PES).

AUTOSURF: A Freely Available Program To Construct Potential Energy Surfaces.

The potential energy surface (PES) of a molecular system constitutes a cornerstone for nearly every theoretical study of spectroscopy and dynamics. We present here AUTOSURF, our freely distributed code for the automated construction of PESs. This first release treats van der Waals systems composed of two rigid fragments.

Theoretical study of HCN-water interaction: five dimensional potential energy surfaces.

A new five-dimensional potential energy surface is calculated at the coupled-cluster CCSD(T) level of theory for the HCN-water system, treating both monomers as rigid rotors. The associated methodology, which combines extensive ab initio calculations of moderate accuracy (CCSD(T)/AVDZ) and a fitting procedure involving a much lower angular coverage with more accurate ab initio calculations (CCSD(T)/CBS), is described in detail. This methodology provides a time-saving approach to compute quantitatively accurate potential energy surfaces with reasonable computational effort.

New potential energy surface for the HCS(+)-He system and inelastic rate coefficients.

A new high quality potential energy surface is calculated at a coupled-cluster single double triple level with an aug-cc-pV5Z basis set for the HCS(+)-He system. This potential energy surface is used in low energy quantum scattering calculations to provide a set of (de)-excitation cross sections and rate coefficients among the first 20 rotational levels of HCS(+) by He in the range of temperature from 5 K to 100 K. The paper discusses the impact of the new ab initio potential energy surface on the cross sections at low energy and provides a comparison with the HCO(+)-He system.