Accurate Potential Energy Surface and Calculated Spectroscopic Properties for CdH2 Isotopomers

The potential energy function and electric dipole moment function (EDMF) are computed for ClO X 2∏ using several different techniques to include electron correlation. The EDMF is used to compute Einstein coefficients, vibrational lifetimes, and dipole moments in higher vibrational levels. Remaining questions concerning the position of the maximum of the EDMF may be resolved through experimental measurement of dipole moments of higher vibrational levels. The band strength of the 1–0 fundamental transition is computed to be 12±2 cm−2 atm−1 in good agreement with three experimental values, but larger than a recent value of 5 cm−2 atm−1 determined from infrared heterodyne spectroscopy. The theoretical methods used include SCF, CASSCF, multireference singles plus doubles configuration interaction (MRCI) and contracted CI, coupled pair functional (CPF), and a modified version of the CPF method. The results obtained using the different methods are critically compared.

The high resolution rovibrational spectrum of H2S has been evaluated from three-dimensional ab initio potential energy and electric dipole moment functions and variational rovibrational eigenfunctions, which took full account of anharmonicity effects and rotation–vibration coupling. The quality of the near equilibrium theoretical potential energy function has been checked by comparisons with experimental equilibrium structure, empirical quartic force fields, vibrational band origins, centrifugal distortion constants, and rotational energy levels. All parameters agree well with the available experimental data. Vibrational band intensities for the ν2, 2ν2, ν1, and ν3 bands have been calculated from empirical and ab initio dipole moment functions and compared with experimental and theoretical integrated band intensities. The difficulties arising by the derivation of such data from the experimental intensities of H2S are discussed. The theoretical results strongly suggest that higher than first derivatives are needed for a proper description of the dipole moment function. The room temperature absorption spectra have been evaluated ab initio for the pure rotational and the ν2, 2ν2, ν1, and ν3 transitions. The unusual intensity pattern of the P, Q, and R branches attributed to the rotational–vibrational coupling has been well reproduced. Absolute line intensities calculated previously by perturbation theory are compared with variational results. The purely theoretical line intensities agree satisfactorily with experiment for the bending transitions, however, the extremely flat regions of the dipole moment functions along the bond stretching displacements make the transition intensities very sensitive to the values of the dipole moment derivatives.

We obtain an accurate methane potential energy surface (PES) by starting with the ab initio PES of Schwenke and Partridge [Spectrochim. Acta A 57, 887 (2001)] and adjusting 5 of their parameters to reproduce 40 reliable experimentally determined vibrational levels of CH4. The 40 levels include all 35 levels in polyads up to and including the Octad and 5 levels in the Tetradecad. The Tetradecad levels are obtained from direct experimental transitions. The fit reduces the root mean square deviation of these 40 levels from 4.80 cm(-1) to 0.28 cm(-1). The new PES ought to aid in the analysis of the Tetradecad. To further test the accuracy of the new PES, vibrational levels are computed for CH4, CH3D, CHD3, and (13)CH4 and are compared with the extensive experimental data. The errors are all within about 1 cm(-1) except for a few cases.

ABSTRACTSpectroscopically accurate Potential Energy Surfaces (PESs) are fundamental for explaining and making predictions of the infrared and microwave spectra of van der Waals (vdW) complexes, and the model used for the potential energy function is critically important for providing accurate, robust and portable analytical PESs. The Morse/Long-Range (MLR) model has proved to be one of the most general, flexible and accurate one-dimensional (1D) model potentials, as it has physically meaningful parameters, is flexible, smooth and differentiable everywhere, to all orders and extrapolates sensibly at both long and short ranges. The Multi-Dimensional Morse/Long-Range (mdMLR) potential energy model described herein is based on that 1D MLR model, and has proved to be effective and accurate in the potentiology of various types of vdW complexes. In this paper, we review the current status of development of the mdMLR model and its application to vdW complexes. The future of the mdMLR model is also discussed. This review can serve as a tutorial for the construction of an mdMLR PES.

A new hot line list for the main isotopologue of CO2, 12C16O2 is presented. The line list consists of almost 2.5 billion transitions between 3.5 million rotation-vibration states of CO2 in its ground electronic state, covering the wavenumber range 0–20 000 cm−1 (λ > 0.5 µm) with the upper and lower energy thresholds of 36 000 cm−1 and 16 000 cm−1, respectively. The ro-vibrational energies and wavefunctions are computed variationally using the accurate empirical potential energy surface Ames-2. The ro-vibrational transition probabilities in the form of Einstein coefficients are computed using an accurate ab initio dipole moment surface with variational program TROVE. A new implementation of TROVE which uses an exact nuclear-motion kinetic energy operator is employed. Comparisons with the existing hot line lists are presented. The line list should be useful for atmospheric retrievals of exoplanets and cool stars. The UCL-4000 line list is available from the CDS and ExoMol data bases.

Nonadiabatic wave packet dynamics on the coupled [formula omitted]/ [formula omitted] electronic states of NO 2 based on new ab initio potential energy surfaces

The reaction between H and benzene is a prototype for reactions of radicals with aromatic hydrocarbons. Here we report calculations of the reaction rate constants and the branching ratios of the two channels of the reaction (H addition and H abstraction) over a wide temperature and pressure range. Our calculations, obtained with an accurate potential energy surface, are based on variational transition-state theory for the high-pressure limit of the addition reaction and for the abstraction reaction and on system-specific quantum Rice-Ramsperger-Kassel theory calibrated by variational transition-state theory for pressure effects on the addition reaction. The latter is a very convenient way to include variational effects, corner-cutting tunneling, and anharmonicity in falloff calculations. Our results are in very good agreement with the limited experimental data and show the importance of including pressure effects in the temperature interval where the mechanism changes from addition to abstraction. We found a negative temperature effect of the total reaction rate constants at 1 atm pressure in the temperature region where experimental data are missing and accurate theoretical data were previously missing as well. We also calculated the H + C6H6/C6D6 and D + C6H6/C6D6 kinetic isotope effects, and we compared our H + C6H6 results to previous theoretical data for H + toluene. We report a very novel nonmonotonic dependence of the kinetic isotope effect on temperature. A particularly striking effect is the prediction of a negative temperature dependence of the total rate constant over 300-500 K wide temperature ranges, depending on the pressure but generally in the range from 600 to 1700 K, which includes the temperature range of ignition in gasoline engines, which is important because aromatics are important components of common fuels.

The reaction OH + HO2→ H2O + O2 is of great significance in interstellar media, the atmosphere, and combustion. In addition, it presents a prototypical reaction between two non-atom radical species. However, the temperature dependence of its rate coefficients has been debated for several decades. In this work, the rate coefficients are revisited by the quasi-classical trajectory (QCT) approach. To this end, a globally accurate full-dimensional potential energy surface of the ground triplet state for the title reaction is constructed using the permutation invariant polynomial-neural network (PIP-NN) method based on 108 000 points calculated at the level of CCSD(T)-F12a/AVTZ, in which particular attention is paid to the initial guess in the preceding Hartree-Fock procedure to obtain reliable ab initio energies. The QCT rate coefficients are compared to available experimental and theoretical results. It has been found that not only the trend, but also the magnitude, i.e. the large negative temperature dependence at low temperatures, and slightly positive temperature dependence at high temperatures, are consistent with some experiments.

A new diabatization method is presented, which is suitable for the development of accurate high-dimensional coupled potential energy surfaces for use in quantum dynamics studies. The method is based on the simultaneous use of adiabatic wave function and energy data, respectively, and combines block-diagonalization and diabatization by ansatz approaches. It thus is called hybrid diabatization. The adiabatic wave functions of suitable ab initio calculations are projected onto a diabatic state space and the resulting vectors are orthonormalized like in standard block-diagonalization. A parametrized diabatic model Hamiltonian is set up as an ansatz for which the block-diagonalization data can be utilized to find the optimal model. Finally, the parameters are optimized with respect to the ab initio reference data such that the deviations between adiabatic energies and eigenvalues of the model as well as projected state vectors and eigenvectors of the model are minimized. This approach is particularly advantageous for problems with a complicated electronic structure where the diabatic state space must be of higher dimension than the number of calculated adiabatic states. This is an efficient way to handle problems with intruder states, which are very common for reactive systems. The use of wave function information also increases the information content for each data point without additional cost, which is beneficial in handling the undersampling problem for high-dimensional systems. The new method and its performance are demonstrated by application to three prototypical systems, ozone (O3), methyl iodide (CH3I), and propargyl (H2CCCH).

The high resolution spectroscopy of the SO2 molecule is of great topical interest, in a wide variety of contexts ranging from origins of higher life, to astrophysics of the interstellar medium, to environmental chemistry. In particular, the C̃1B2 ← X̃1A1 UV photoabsorption spectrum has received considerable attention. This spectrum exhibits a highly regular progression of ∼20 or so strong peaks, spaced roughly 350 cm-1 apart, which is comparable to the C̃1B2 bending vibrational frequency. Accordingly, they have for decades been largely attributed to the (1, v2', 2) ← (0, 0, 0) bend progression. Using a highly accurate new ab initio potential energy surface (PES) for the C̃1B2 state, we compute vibrational energy levels and wave functions, and compare with a photoabsorption calculation obtained using the same PES and corresponding C̃1B2 ← X̃1A1 transition dipole surface (TDS). We find that the above putative assignment is incorrect, contradicting even general qualitative trends-thus necessitating a very different dynamical picture for this highly unusual molecule.

The performance of vibrational structure calculations beyond harmonic approximation in the framework of the vibrational self-consistent field method with second-order perturbation corrections (VSCF-PT2) is investigated in conjunction with very accurate potential energy surfaces (PESs) given by various coupled-cluster electronic structure theories. The quality of anharmonic calculations depends on the accuracy of the underlying multidimensional PES obtained from its functional form, which is given by the level of electronic structure theory. Two such highest levels of typical coupled-cluster electronic structure methods, CCSD and the ″gold standard″ CCSD(T), along with their variants such as CCD, CR-CCL (completely renormalized CR-CC(2,3) approach), and CCSD(TQ) are tested for the construction of accurate anharmonic potentials without any fitting or ad hoc scaling and using cc-pVTZ basis sets. The accuracy of VSCF-PT2 theory in comparison to experimental values is tested for a series of 16 molecules with 135 fundamental bands, 64 overtones, and combination bands and also for 39 intensities. It is found that CCD and CCSD bind the potential tighter than CCSD(T) and the computed VSCF-PT2 transitions are more blue-shifted showing higher deviation from the experiment. In general, VSCF-PT2 results computed at the CCSD(T) potential offer a good cost/accuracy ratio, with the mean absolute deviation and the mean absolute percentage error with the experiment being ∼16 cm-1 and 1.38, respectively, for fundamentals. Additionally, while the CR-CCL and CCSD(TQ) methods offer similar levels of accuracies as compared to CCSD(T), the former offers a better accuracy/cost ratio than the latter and is a suitable alternative to CCSD(T).

The accuracy of three-dimensional adiabatic and diabatic potential energy surfaces is calculated using ab initio methods and is numerically fitted for the two lowest electronic states 1 and 22A′ of the LiH2 system, which are very important for the Li (2p) + H2 reaction. The finite difference method is performed to generate the mixing angles, which are used to educe the diabatic potential from the adiabatic potential. The accurate conical intersection (CI) is studied in this work with three different basis sets. The energy of the conical intersection is slightly lower (nearly 0.12 eV) than that of the perpendicular intermediate on the first excited state. By analyzing the potential energy surfaces in this work we can suggest that the most possible reaction pathway for the title reaction is Li (2p) + H2 → LiH2 (22A′) (C2v) → CI → LiH2 (12A′) (C2v) → LiH⋯H → LiH (X1∑g+) + H. The conical intersection and (22A′) intermediate may play a vital role in the title reaction.

The viscomagnetic effect for two linear molecules, N2 and CO2, has been calculated in the dilute-gas limit directly from the most accurate ab initio intermolecular potential energy surfaces presently available. The calculations were performed by means of the classical trajectory method in the temperature range from 70 K to 3000 K for N2 and 100 K to 2000 K for CO2, and agreement with the available experimental data is exceptionally good. Above room temperature, where no experimental data are available, the calculations provide the first quantitative information on the magnitude and the behavior of the viscomagnetic effect for these gases. In the presence of a magnetic field, the viscosities of nitrogen and carbon dioxide decrease by at most 0.3% and 0.7%, respectively. The results demonstrate that the viscomagnetic effect is dominated by the contribution of the jj¯ polarization at all temperatures, which shows that the alignment of the rotational axes of the molecules in the presence of a magnetic field is primarily responsible for the viscomagnetic effect.

Chemisorption of H and H2 on clean W(100) and W(110) surfaces is investigated from extensive density functional theory (DFT) calculations within the generalized gradient approximation (GGA). We obtain properties of the clean surfaces [e.g., the well-known reconstructed structure of W(100) below 200 K] as well as adsorption energies and geometries of H atoms chemisorbed on both faces of W in very good agreement with available experimental data. From our DFT-GGA results, we build accurate six-dimensional potential energy surfaces (PESs) which are used to compute dissociative sticking probabilities of H2 on both faces of W through classical trajectory calculations. For both systems, our theoretical results agree with the available experimental data at low impact energies and are larger than experiments at higher energies. The main signatures of the sticking probabilities found in molecular beam experiments are reproduced, at least qualitatively, by our calculations: for instance, (i) the larger values for W(100) than for W(110), (ii) the nonmonotonic and monotonic energy variation for W(100) and W(110), respectively, and (iii) the dependence on incidence angle as a function of W face and impact energy. Our calculations show that dissociative adsorption of H2 on both W(100) and W(110) is a nonactivated process. In contrast with the widely accepted assumption that H2 chemisorption on W(110) is a purely direct process, we obtain that, at very low energies, adsorption takes place through an indirect (dynamic trapping) and a direct mechanism on both surfaces. Thus, the qualitatively different behavior of the sticking probability at low energies arises from a smaller contribution of dynamic trapping in the case of H2/W(110), due to slight differences between the corresponding PESs far from the surface in the entrance channel.

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