On the use of explicitly correlated treatment methods for the generation of accurate polyatomic –He/H2 interaction potential energy surfaces: The case of C3–He complex and generalization

Bending Relaxation of H2 O by Collision with Para- and Ortho-H2

We extend our recent theoretical work on the bending relaxation of H2 O in collisions with H2 by including the three water modes of vibration coupled with rotation, as well as the rotation of H2 . Our full quantum close-coupling method (excluding the H2 vibration) is combined with a high-accuracy nine-dimensional potential energy surface. The collisions of para-H2 O and ortho-H2 O with the two spin modifications of H2 are considered and compared for several initial states of H2 O. The convergence of the results as a function of the size of the rotational basis set of the two colliders is discussed. In particular, near-resonant energy transfer between H2 O and H2 is found to control the vibrational relaxation process, with a dominant contribution of transitions withΔ j 2 = j 2 f - j 2 i ${{\rm{\Delta }}j_2 = j_2^f - j_2^i }$ =+ 2 , + 4 ${ + 2, + 4}$ ,j 2 i ${j_2^i }$ andj 2 f ${j_2^f }$ being respectively the H2 initial and final rotational quantum numbers. Finally, the calculated value of the H2 O bending relaxation rate coefficient at 295 K is found to be in excellent agreement with its experimental estimate.

Towards the generation of potential energy surfaces of weakly bound medium-sized molecular systems: the case of benzonitrile-He complex

Currently, the explicitly correlated coupled cluster method is used routinely to generate the multi-dimensional potential energy surfaces (mD-PESs) of van der Waals complexes of small molecular systems relevant for atmospheric, astrophysical and industrial applications. Although very accurate, this method is computationally prohibitive for medium and large molecules containing clusters. For instance, the recent detections of complex organic molecules (COMs) in the interstellar medium, such as benzonitrile, revealed the need to establish an accurate enough electronic structure approach to map the mD-PESs of these species interacting with the surrounding gases. As a benchmark, we have treated the case of the polar molecule benzonitrile interacting with helium, where we use post-Hartree-Fock and symmetry-adapted perturbation theory (SAPT) techniques. Accordingly, we show that MP2 and distinguishable-cluster approximation (DCSD) cannot be used for this purpose, whereas accurate enough PESs may be obtained using the corresponding explicitly correlated versions (MP2-F12 or DCSD-F12) with a reduction in computational costs. Alternatively, computations revealed that SAPT(DFT) is as performant as CCSD(T)-F12/aug-cc-pVTZ, making it the method of choice for mapping the mD-PESs of COMs containing clusters. Therefore, we have used this approach to generate the 3D-PES of the benzonitrile-He complex along the intermonomer Jacobi coordinates. As an application, we have incorporated the analytic form of this PES into quantum dynamical computations to determine the cross sections of the rotational (de-)excitation of benzonitrile colliding with helium at a collision energy of 10 cm-1.

Electronic structure calculations and quantum dynamics of rotational deexcitation of CNNC by He

The quantum dynamics of rotational transitions of the diisocyanogen (CNNC) molecule undergoing collision with the helium (He) atom occurring in the interstellar medium (ISM) has been studied. The rotational deexcitation cross sections are extracted by first computing an ab initio potential energy surface of CNNC-He using the coupled-cluster with single and double and perturbative triple excitations with the F12a method (CCSD(T)-F12a) employing the aug-cc-pVTZ basis set. Utilizing the multipole expansions, collisional cross sections are determined for total energies of up to 1000 cm^-1 by the close coupling equations. The discussion on propensity rules suggests that the transitions have even Δj values, while odd Δj valued transitions are forbidden due to C and N nuclei spin statistics. Quasi-bound states present in the CNNC-He van der Waals complex resulted in the resonances coming from the rapid oscillations in the values of the cross sections in the region of low energy. Rotational deexcitation rate coefficients are further worked out by averaging the calculated cross sections at temperatures below 200 K. The new findings of the study will be beneficial in modeling the abundance of diisocyanogen in the ISM.

A close coupling study of the bending relaxation of H2O by collision with He

We present a close coupling study of the bending relaxation of H₂O by collision with He, taking explicitly into account the bending-rotation coupling within the rigid-bender close-coupling method. A 4D potential energy surface is developed based on a large grid of ab initio points calculated at the coupled-cluster single double triple level of theory. The bound states energies of the He-H₂O complex are computed and found to be in excellent agreement with previous theoretical calculations. The dynamics results also compare very well with the rigid-rotor results available in the Basecol database and with experimental data for both rotational transitions and bending relaxation. The bending-rotation coupling is also demonstrated to be very efficient in increasing bending relaxation when the rotational excitation of H₂O increases.

Explicitly correlated potential energy surface of the CH3Cl-He van der Waals complex and applications

A new 3D-potential energy surface (3D-PES) for the weakly bound CH₃Cl-He complex is mapped in Jacobi coordinates. Electronic structure calculations are performed using the explicitly correlated coupled clusters with single, double, and perturbative triple excitations approach in conjunction with the aug-cc-pVTZ basis set. Then, an analytical expansion of this 3D-PES is derived. This PES shows three minimal structures for collinear C-Cl-He arrangements and for He located in between two H atoms, in the plane parallel to the three H atoms, which is near the center of mass of CH₃Cl. The latter form corresponds to the global minimum. Two maxima are also found, which connect the minimal structures. We then evaluated the pressure broadening coefficients of the spectral lines of CH₃Cl in a helium bath based on our ab initio potential. Satisfactory agreement with experiments was observed, confirming the good accuracy of our 3D-PES. We also derived the bound rovibronic levels for ortho- and para-CH₃Cl-He dimers after quantum treatment of the nuclear motions. For both clusters, computations show that although the ground vibrational state is located well above the intramolecular isomerization barriers, the rovibronic levels may be associated with a specific minimal structure. This can be explained by vibrational localization and vibrational memory effects.

Rotational (de-)excitation of isocyanogen by collision with helium at low energies

Isocyanogen, CNCN, was discovered very recently in the interstellar medium (ISM). At present, the rate coefficients for the rotational (de-)excitation of CNCN by collisions with He are determined. First, we mapped the interaction potential between CNCN and He in Jacobi coordinates using highly correlated ab initio methodology. Then, an analytical expansion of the CNCN-He potential energy surface is derived. Later on, quantum dynamical treatments of nuclear motions are performed using the close coupling technique. We obtained the cross sections for the rotational (de-)excitation of CNCN after a collision by He up to 2000 cm^-1 total energies. These cross sections are used to deduce the collision rates in the 10-300 K range. These data are needed for modeling the CNCN abundances in the ISM. This work should help for determining the abundance of such non-symmetrical dicyanopolyynes in astrophysical media and indirectly the symmetric one [Cyanogen (NCCN)].

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. Our potential reproduces the main features reported in the literature, and will allow us to perform the first quantum and semi-classical simulations of the collisional dynamic on this system.

Microscopic investigations of site and functional selectivity of triazole for CO2 capture and catalytic applications

Ab initio and DFT studies on CO₂ interacting with different tautomers and isomers of triazole (TZ) are carried out to understand the adsorption mechanism, site selectivity and their mutual preferential attracting sites.

Potential energy surface of the CO2-N2 van der Waals complex

Four-dimensional potential energy surface (4D-PES) of the atmospherically relevant CO2-N2 van der Waals complex is generated using the explicitly correlated coupled cluster with single, double, and perturbative triple excitation (CCSD(T)-F12) method in conjunction with the augmented correlation consistent triple zeta (aug-cc-pVTZ) basis set. This 4D-PES is mapped along the intermonomer coordinates. An analytic fit of this 4D-PES is performed. Our extensive computations confirm that the most stable form corresponds to a T-shape structure where the nitrogen molecule points towards the carbon atom of CO2. In addition, we located a second isomer and two transition states in the ground state PES of CO2-N2. All of them lay below the CO2 + N2 dissociation limit. This 4D-PES is flat and strongly anisotropic along the intermonomer coordinates. This results in the possibility of the occurrence of large amplitude motions within the complex, such as the inversion of N2, as suggested in the recent spectroscopic experiments. Finally, we show that the experimentally established deviations from the C2v structure at equilibrium for the most stable isomer are due to the zero-point out-of-plane vibration correction.