Collisional excitation of propargylimine by helium: new ab initio 3D-potential energy surfaces and scattering calculations.

We present quantum coupled-state calculations for the rotational excitation of interstellar propargylimine due to collisions with helium. The calculations are based on new high-accurate three-dimensional potential energy surfaces (3D-PESs) adapted for rigid-rotor scattering computations. The two PESs (Z/E-PGIM-He) were determined using the explicitly correlated coupled-cluster approach with single, double and perturbative triple excitation [CCSCD(T)-F12] and the standard aug-cc-pVTZ basis set. These PESs present many minima with a global minimum of -47.61 cm-1 for Z-PGIM-He and -54.16 cm-1 for E-PGIM-He. While the PESs for both complexes are qualitatively similar, that of E-PGIM-He is more anisotropic. The state-to-state collisional cross-section calculations are performed for all rotational levels J ≤ 12 with energies below Erot = 30 cm-1 and for total energies up to 500 cm-1. The corresponding collisional rate coefficients are derived for kinetic temperatures up to 120 K. A propensity rule is seen, for rotational excitation cross sections and de-excitation rate coefficients, that favors even ΔJ transitions but with different orders of magnitude. We expect that the retrieved results will contribute to improving atmospheric and astrophysics models.

New theoretical investigation of rotational inelastic (de)-excitation of calcium isocyanide CaNC(2Σ+) in collision with He(1S).

The theoretical study of collisions between atoms and molecules provides a detailed description of the involved mechanisms and greatly contributes to improving atmospheric and astrophysics models. In the present paper, we focus on the new calculation of rate coefficients for the first 25 rotational levels of the CaNC molecule in collision with He. A new 2D potential energy surface (2D-PES), for the CaNC-He system, was determined using the single, double and perturbative triple excitation restricted coupled-cluster method [rccsd(t)] and the standard aug-cc-pVQZ basis sets. This PES presents a global minimum with a well depth of -21.93 cm-1 located at R = 8a0 and γ = 116°. State-to-state collisional excitation cross-sections of the fine-structure levels of CaNC(2Σ+)-He are calculated for energies up to 305 cm-1, which yield, after thermal averaging, rate coefficients up to 70 K. A ΔJ = ΔN propensity rule was observed. A comparison of the CaNC rates with those of valence isoelectronic MgNC has been investigated. These new data are necessary for the CaNC abundance determination and interpretation of its observed lines.

Potential Energy Surface for the CH4-H2 van der Waals Interaction.

Methane is an ubiquitous molecule, present as a minor component in many environments, including the Earth and planet atmospheres. Its van der Waals interaction with the main gases is an important ingredient for the understanding of radiative properties for those atmospheres. We present here the first precise determination of the interaction between CH4 and H2. We compute the interaction in an ab initio coupled cluster formalism, with extended atomic bases. We compare a pure CCSD(T) approach to an explicitly correlated CCSD(T)-F12a formalism. The full geometry of scattering two rigid molecules is used, resulting in a potential energy surface depending on 5 degrees of freedom. The long-range part of the ab initio computation is compared with an analytic multipolar expansion. The potential is fit onto a suitable formalism for subsequent scattering dynamics. The potential well is computed at -92.92 cm-1, an intermediate value if compared with other methane-diatomic molecule interactions.

Rotational (de)-excitation of linear C3O by collision with He.

Tricarbon monoxide (C3O) is an astrochemically important molecule. It is a probe element for determining the chemical composition of gases in molecular clouds, as C3O is a major intermediate of ion-molecule reactions in the gas phase. For C3O, no collision coefficients are available in the literature. To estimate the abundance of C3O in a solar cold dark cloud: Taurus Molecular Cloud 1 (TMC-1) and find the range of cloud parameters, some authors used the calculated values for the HC3N molecule, which is isoelectronic and has similar rotational constants. Indeed, the calculation of the rate coefficients of C3O(1Σ+) induced by collision with He is performed for thermal temperatures below 25 K. These calculations are based on a new two-dimensional potential energy surface obtained from the explicitly correlated coupled cluster with a single, double and perturbative triple excitation (ccsd(t)-f12) ab initio approach associated with aug-cc-pVTZ basis sets. The PES was found to have a global minimum at (R = 6.2 Bohr and θ = 73°) with a depth of -53.4 cm-1 below the C3O-He dissociation limit. Using this PES, the integral cross sections are performed in the close-coupling quantum time independant formalism for Ec ≤ 110 cm-1 and J ≤ 12. These cross sections were then averaged at low temperature to obtain the downward rate coefficients. The new collisional data should significantly help the interpretation of interstellar C3O emission lines observed with current and future telescopes. We expect that they will allow the accurate determination of the C3O abundance in the interstellar medium, which is crucial to understand the chemistry of carbon chain species in the interstellar gas.

Dynamics of AlOH inelastic scattering with p-H2(J = 0) on a full and accurate potential energy surface.

Alkali aluminum hydroxide, AlOH, has associated features with the chemistry of aluminum-bearing species and generally with metal hydroxide molecules in the interstellar clouds where it has been observed. The aim of this work is to obtain accurate rotational rate coefficients of AlOH colliding with the most abundant molecular species in the interstellar medium (ISM), H2, in its lowest rotational state p-H2(J = 0) for the kinetic temperature range 5-80 K. A full and accurate 4D-PES was generated using explicitly correlated coupled cluster with single, double and perturbative triple excitation CCSD(T)-F12a augmented by the aVTZ basis set. Both the close coupling CC and coupled states CS techniques were used to generate rotational cross-sections for AlOH-p-H2 including 13 rotational states (J1 = 0,…,12) for the AlOH molecule and the J2 = 0 state for the H2 molecule for energy thresholds up to E = 500 cm-1. Propensity rules that favor odd ΔJ1 transitions are found all over the temperature range. Further, a comparison of the present AlOH-p-H2(J = 0) rate coefficients with scaled AlOH-He rates was made, revealing mainly good agreement with some discrepancies appearing for large ΔJ1. The use of the present rates is viewed to be a good tool to estimate the aluminum hydroxide abundance.

van der Waals interaction of HNCO and H2: Potential Energy Surface and Rotational Energy Transfer.

Isocyanic acid (HNCO) is the most stable of all its isomers; it has been observed repeatedly in many different conditions of the Interstellar Media, and its chemistry is poorly known. To quantitatively estimate the abundance of HNCO with respect to other organic molecules, we compute its rotational quenching rates colliding with H2, the most common gas in the gaseous Interstellar Media. We compute ab initio the van der Waals interaction HNCO-H2, in the rigid molecules approximation, with a CCSD(T)-F12a method. On the fitted ab initio surface, inelastic scattering cross sections and rates are calculated for a temperature range of 7-200 K, with the coupled-states quantum time-independent formalism. The critical densities are high enough to yield rotational temperatures of HNCO differing significantly from the kinetic temperature of H2, especially so for the shorter wavelengths observed at the ALMA interferometer. It is found that the quenching rates for collisions with ortho- or para-H2 differ greatly, opening the possibility of far from equilibrium populations of some rotational levels of HNCO.

On the accuracy of explicitly correlated methods to generate potential energy surfaces for scattering calculations and clustering: application to the HCl-He complex.

We closely compare the accuracy of multidimensional potential energy surfaces (PESs) generated by the recently developed explicitly correlated coupled cluster (CCSD(T)-F12) methods in connection with the cc-pVXZ-F12 (X = D, T) and aug-cc-pVTZ basis sets and those deduced using the well-established orbital-based coupled cluster techniques employing correlation consistent atomic basis sets (aug-cc-pVXZ, X = T, Q, 5) and extrapolated to the complete basis set (CBS) limit. This work is performed on the benchmark rare gas-hydrogen halide interaction (HCl-He) system. These PESs are then incorporated into quantum close-coupling scattering dynamical calculations in order to check the impact of the accuracy of the PES on the scattering calculations. For this system, we deduced inelastic collisional data including (de-)excitation collisional and pressure broadening cross sections. Our work shows that the CCSD(T)-F12/aug-cc-pVTZ PES describes correctly the repulsive wall, the van der Waals minimum and long range internuclear distances whereas cc-pVXZ-F12 (X = D,T) basis sets are not diffuse enough for that purposes. Interestingly, the collision cross sections deduced from the CCSD(T)-F12/aug-cc-pVTZ PES are in excellent agreement with those obtained with CCSD(T)/CBS methodology. The position of the resonances and the general shape of these cross sections almost coincide. Since the cost of the electronic structure computations is reduced by several orders of magnitude when using CCSD(T)-F12/aug-cc-pVTZ compared to CCSD(T)/CBS methodology, this approach can be recommended as an alternative for generation of PESs of molecular clusters and for the interpretation of accurate scattering experiments as well as for a wide production of collisional data to be included in astrophysical and atmospherical models.

Rotationally inelastic dynamics of LiH (X(1)Σ(+), v = 0) in collisions with Ar: State-to-state inelastic rotational rate coefficients.

A theoretical study of rotational collision of LiH(X(1)Σ(+),v = 0, J) with Ar has been carried out. The ab initio potential energy surface (PES) describing the interaction between the Ar atom and the rotating LiH molecule has been calculated very accurately and already discussed in our previous work [Computational and Theoretical Chemistry 993 (2012) 20-25]. This PES is employed to evaluate the de-excitation cross sections. The ab initio PES for the LiH(X(1)Σ(+))-Ar((1)S) Van der waals system is calculated at the coupled-cluster [CCSD(T)] approximation for a LiH length fixed to an experimental value of 3.0139 bohrs. The basis set superposition error (BSSE) is corrected and the bond functions are placed at mid-distance between the center of mass of LiH and the Ar atom. The cross sections are then derived in the close coupling (CC) approach and rate coefficients are inferred by averaging these cross sections over a Maxwell-Boltzmann distribution of kinetic energies. The 11 first rotational levels of rate coefficients are evaluated for temperatures ranging from 10 to 300 K. We notice that the de-excitation rate coefficients appear large in the order 10(-10) cm(-3) s(-1) and show very low temperature dependence. The rate coefficients magnify significantly the propensity toward ∆ J = -1 transitions. These results confirm the same propensity already noted for the cross sections.