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.

We present a new four-dimensional (4D) potential energy surface for the HCl-H2 van der Waals system. Both molecules were treated as rigid rotors. Potential energy surface was obtained from electronic structure calculations using a coupled cluster with single, double, and perturbative triple excitations method. The four atoms were described using the augmented correlation-consistent quadruple zeta basis set and bond functions were placed at mid-distance between the HCl and H2 centers of mass for a better description of the van der Waals interaction. The global minimum is characterized by the well depth of 213.38 cm(-1) corresponding to the T-shape structure with H2 molecule on the H side of the HCl molecule. The dissociation energies D0 are 34.7 cm(-1) and 42.3 cm(-1) for the complex with para- and ortho-H2, respectively. These theoretical results obtained using our new PES are in good agreement with experimental values [D. T. Anderson, M. Schuder, and D. J. Nesbitt, Chem. Phys. 239, 253 (1998)]. Close coupling calculations of the inelastic integral rotational cross sections of HCl in collisions with para-H2 and ortho-H2 were performed at low and intermediate collisional energies. Significant differences exist between para- and ortho-H2 results. The strongest collision-induced rotational HCl transitions are the transitions with Δj = 1 for collisions with both para-H2 and ortho-H2. Rotational relaxation of HCl in collision with para-H2 in the rotationally excited states j = 2 is dominated by near-resonant energy transfer.

Context. The freezeout of gas-phase species onto cold dust grains can drastically alter the chemistry and the heating-cooling balance of protostellar material. In contrast to well-known species such as carbon monoxide (CO), the freezeout of various carriers of elements with abundances < 10 −5 has not yet been well studied. Aims. Our aim here is to study the depletion of chlorine in the protostellar core, OMC-2 FIR 4. Methods. We observed transitions of HCl and H 2 Cl + towards OMC-2 FIR 4 using the Herschel Space Observatory and Caltech Submillimeter Observatory facilities. Our analysis makes use of state of the art chlorine gas-grain chemical models and newly calculated HCl-H 2 hyperfine collisional excitation rate coefficients. Results. A narrow emission component in the HCl lines traces the extended envelope, and a broad one traces a more compact central region. The gas-phase HCl abundance in FIR 4 is 9 × 10 −11 , a factor of only 10 −3 that of volatile elemental chlorine. The H 2 Cl + lines are detected in absorption and trace a tenuous foreground cloud, where we find no depletion of volatile chlorine. Conclusions. Gas-phase HCl is the tip of the chlorine iceberg in protostellar cores. Using a gas-grain chemical model, we show that the hydrogenation of atomic chlorine on grain surfaces in the dark cloud stage sequesters at least 90% of the volatile chlorine into HCl ice, where it remains in the protostellar stage. About 10% of chlorine is in gaseous atomic form. Gas-phase HCl is a minor, but diagnostically key reservoir, with an abundance of 10 −10 in most of the protostellar core. We find the [ 35 Cl]/[ 37 Cl] ratio in OMC-2 FIR 4 to be 3.2 ± 0.1, consistent with the solar system value.