A comparative study of CO2–Ar potential surfaces

Simulation of collision-induced absorption spectra based on classical trajectories and ab initio potential and induced dipole surfaces. II. CO2-Ar rototranslational band including true dimer contribution

This paper presents further development of the new semi-classical trajectory-based formalism described in Paper I [Chistikov et al., J. Chem. Phys. 151, 194106 (2019)]. We report the results of simulation and analysis of the low-frequency collision-induced absorption (CIA) in CO₂-Ar, including its true dimer component. Our consideration relies on the use of ab initio intermolecular potential energy and induced dipole surfaces for CO₂-Ar calculated in an assumption of a rigid CO₂ structure using the CCSD(T) method. The theory, the details of which are reported in Paper I [Chistikov et al., J. Chem. Phys. 151, 194106 (2019)], permits taking into account the effect of unbound and quasi-bound classical trajectories on the CIA in the range of a rototranslational band. This theory is largely extended by trajectory-based simulation of the true bound dimer absorption in the present paper. The spectra are obtained from a statistical average over a vast ensemble of classical trajectories restricted by properly chosen domains in the phase space. Rigorous classical theory is developed for two low-order spectral moments interpreted as the Boltzmann-weighted average of the respective dipole functions. These spectral moments were then used to check the accuracy of our trajectory-based spectra, for which both spectral moments can be evaluated independently in terms of specific integrals over the trajectory-based calculated spectral profiles. Good agreement between the spectral moments calculated as integrals over the frequency domain or the phase space largely supports the reliability of our simulated CIA spectra, which conform with the available microwave and far-infrared observations.

Theoretical studies for the infrared spectra of Ar-CO2 complex: Fundamental and combination bands

Two potential energy surfaces (PESs) were constructed for the ground and excited states of Ar-CO₂ complex at the rigid rotor approximation. Besides the notable T-shape structure, two equivalent linear structures were also found on the PES for the first time. Based on the PESs of ground and excited states, the bound state calculations were performed to determine the rotational energy levels for the ground and excited states. In combination of the experimental spectroscopic parameters of ground state and the differences of rotational energy levels, we give a theoretical prediction of infrared spectra including one fundamental band and two combination bands for two isotopomers Ar-¹²C¹⁶O₂ and Ar-¹²C¹⁸O₂ in the ν₃ region of CO₂ monomer. The predicted transition frequencies and spectroscopic parameters of excited states are in excellent agreement with the available experimental data, and these results can also be used as a guide to perform the further investigation for the infrared spectra of combination bands experimentally.

POTENTIAL ENERGY SURFACE, MICROWAVE AND INFRARED SPECTRA OF THE Xe–CO2 COMPLEX FROM AB INITIO CALCULATIONS

We present a new three-dimensional potential energy surface for Xe–CO2 including the Q3 normal mode for the υ3 antisymmetric stretching vibration of the CO2 molecule. Two vibrationally adiabatic potentials with CO2 in both the ground (υ3 = 0) and the first excited (υ3 = 1) states are generated by the integration of this potential over the Q3 coordinate. Each potential is found to have a T-shaped global minimum. The radial DVR/angular FBR method and the Lanczos algorithm are employed to calculate the rovibrational energy levels. The calculated band origin shifts, microwave and infrared spectra based on the two averaged potentials are in good agreement with the available experimental data.

Carbondioxide rare-gas systems: sensitivity of basis sets and double-hybrid density functionals

This study emphasizes on the performance of six newly developed double-hybrid density functionals (DHDF) in explaining the potential energy curves of different carbondioxide rare-gas systems. The basis set sensitivity has also been explored with the use of three basis sets. Our results suggest that for lighter He/Ne-CO(2) complexes, proper choice of DHDF and basis set lead to results those matches exactly with earlier calculations and also with the experiment. On the other hand, for heavier Ar/Kr-CO(2) complexes although the equilibrium separation distance matches exactly with earlier observations, the interaction energy values lie far apart. The overall investigation emphasizes on the fact that one has to tune the methods and basis sets properly to achieve good and satisfactory results.

A new potential energy surface and predicted infrared spectra of the Ar-CO(2) van der Waals complex

A new potential energy surface for Ar-CO(2) is constructed at the coupled-cluster singles and doubles with noniterative inclusion of connected triple [CCSD(T)] level with augmented correlation-consistent triple-zeta (aug-cc-pVTZ) basis set plus midpoint bond functions. The Q(3) normal mode for the v(3) antisymmetric stretching vibration of CO(2) is involved in the construction of the potential. Effective two-dimensional potentials with CO(2) in the ground and first excited v(3) vibrational states are obtained by averaging a three-dimensional potential for each case over the Q(3) asymmetric stretch vibrational coordinate. Both potentials have only a T-shaped minimum with a well depth of 200.97 and 201.37 cm(-1), respectively. No linear local minima are detected. The radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm are employed to calculate the related rovibrational energy levels. The calculated band origin shift of the complex agrees very well with the observed one (-0.474 versus -0.470 cm(-1)). In addition, the predicted infrared spectra based on the two averaged potentials are in excellent agreement with the available experimental data, which again testifies the accuracy of the new potentials.

Structures and energetics of CO2–Arn clusters (n = 1–21) based on a non-rigid potential model

Energetics and possible stable structures of CO2–Arn (n = 1–21) clusters are investigated by performing molecular-dynamics simulations. The pairwise-additive approximation is tested to construct the potential energy function for describing the non-rigid particle interactions in the system. A potential model by Pariseau et al. (Journal of Chemical Physics, Vol. 42, p. 2335, 1965) is used for the internal motion of the CO2 molecule and the Billing form potential (Chemical Physics, Vol. 185, p. 199, 1994) is used for all other pair interactions. The stable configurations are determined for the ground state of CO2–Arn clusters, and the growing pattern process of the clusters is determined via rearrangement collisions. Ar atoms tend to surround the CO2 molecule, and the clusters prefer to form three-dimensional compact structures. Obtained structures and energetics are in quantitative agreement with previous results (Journal of Chemical Physics, Vol. 109, p. 1343, 1998) that have used split-repulsion and ab initio potentials in which the molecule was treated as rigid.Key words: argon, CO2, cluster, potential energy function, molecular dynamics.

Molecular reorientation of CD(4) in gas-phase mixtures

Spin-lattice relaxation times were measured for the deuterons in CD(4) in pure gas and in mixtures with the following buffer gases: Ar, Kr, Xe, HCl, N(2), CO, CO(2), CF(4), and SF(6). Effective collision cross sections sigma(theta, 2) for the molecular reorientation of CD(4) in collisions with these ten molecules are obtained as a function of temperature. These cross sections are compared with the corresponding cross sections sigma(J) obtained from (1)H spin-rotation relaxation in mixtures of CH(4) with the same set of buffer gases. Various classical reorientation models typically applied in liquids predict different ratios of the reduced correlation times for the reorientation of spherical tops. The Langevin model comes closest to predicting the magnitude of the sigma(theta, 2)/sigma(J) ratio that we obtain for CD(4).

Calculation of the transport properties of carbon dioxide. III. Volume viscosity, depolarized Rayleigh scattering, and nuclear spin relaxation

Transport properties of pure carbon dioxide have been calculated from the intermolecular potential using the classical trajectory method. Results are reported in the dilute-gas limit for volume viscosity, depolarized Rayleigh scattering, and nuclear spin relaxation for temperatures ranging from 200 to 1000 K. Three recent carbon dioxide potential energy hypersurfaces have been investigated. Calculated values for the rotational collision number for all three intermolecular surfaces are consistent with the measurements and indicate that the temperature dependence of the Brau-Jonkman correlation is not applicable for carbon dioxide. The results for the depolarized Rayleigh scattering cross section and the nuclear spin relaxation cross section show that calculated values for the generally more successful potentials differ from the observations by 9% at about 290 K, although agreement is obtained for nuclear spin relaxation at about 400 K.

Classical analysis of intermolecular potentials for Ar–CO2 rotational collisions

Classical trajectory calculations have been performed for four potential energy functions to describe Ar–CO2 collisions. A comparison is given between classical cross sections calculated using the two most recent potential surfaces and two older intermolecular potential surfaces based on the electron gas model. The two-dimensional atom ellipsoid model has also been applied for the study of multiple collisions. The model was able to predict such a phenomenon in agreement with quantum scattering results previously published for an ab initio potential surface in the region of very low collision energy. On the other hand, the two older potentials showed multiple collision effects at very high energies. The comparison of the cross sections showed some deviations from the experimental data. By introducing two parameters, a modified surface is proposed by changing the most recent intermolecular potential. In this case the agreement with experimental measurements and theoretical scattering cross sections was considerably improved. It is concluded that global potential surfaces for describing Ar–CO2 interaction are not well established. To achieve the requirement of reproducing all properties of this system, the present work suggests that one needs further experimental and theoretical investigations. Key words: classical trajectories, dynamics, cross sections, Ar–CO2 collisions, potentials.