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In this work, high-fidelity full-dimensional potential energy surfaces (PESs) of the ground ( X̃2 A′) and first doublet excited ( Ã2 A″) electronic states of HCO were constructed using neural network method. In total, 4624 high-level ab initio points have been used which were calculated at Davidson corrected internally contracted MRCI-F12 level of theory with a quite large basis set (ACV5Z) without any scaling scheme. Compared with the results obtained from the scaled PESs of Ndengué et al., the absorption spectrum based on our PESs has slightly larger intensity, and the peak positions are shifted to smaller energy for dozens of wavenumbers. It is indicated that the scaling of potential energy may make some unpredictable difference on the dynamical results. However, the resonance energies based on those scaled PESs are slightly closer to the current available experimental values than ours. Nevertheless, the unscaled high-level PESs developed in this work might provide a platform for further experimental and theoretical photodissociation and collisional dynamic studies for HCO system.

Full-dimensional adiabatic potential energy surfaces of the electronic ground state X̃ and nine excited states Ã, Ĩ, B̃, C̃, D̃, D̃′, D̃″, Ẽ′ and F̃ of H2O molecule are developed at the level of internally contracted multireference configuration interaction with the Davidson correction. The potential energy surfaces are fitted by using Gaussian process regression combining permutation invariant polynomials. With a large selected active space and extra diffuse basis set to describe these Rydberg states, the calculated vertical excited energies and equilibrium geometries are in good agreement with the previous theoretical and experimental values. Compared with the well-investigated photodissociation of the first three low-lying states, both theoretical and experimental studies on higher states are still limited. In this work, we focus on all the three channels of the highly excited state, which are directly involved in the vacuum ultraviolet photodissociation of water. In particular, some conical intersections of D̃–Ẽ′, Ẽ′-F̃, ÖĨ and Ĩ– C̃ states are clearly illustrated for the first time based on the newly developed potential energy surfaces (PESs). The nonadiabatic dissociation pathways for these excited states are discussed in detail, which may shed light on the photodissociation mechanisms for these highly excited states.

We discuss the details of a time-independent quantum mechanical method and its implementation for full-dimensional non-reactive scattering between a closed-shell triatomic molecule and a closed-shell atom. By solving the time-independent Schrödinger equation within the coupled-channel framework using a log-derivative method, the state-to-state scattering matrix ( S-matrix) can be determined for inelastic scattering involving both the rotational and vibrational modes of the molecule. Various approximations are also implemented. The ABC+D code provides an important platform for understanding an array of physical phenomena involving collisions between atoms and molecules.

While the rigorous time-independent close-coupling approach is ideally suited for cold and ultracold rovibrationally inelastic collision, its application beyond atom–diatom systems in full dimensionality is numerically expensive. Coupled-states (CS) approximation and its extensions are good choices to reduce the computational cost and have been successfully applied to diatom–diatom systems. In this work, we introduce the extended CS (ECS) approximation, in which one or a few nearest Coriolis coupled helicity channels are included. Its usefulness in atom–triatom systems is demonstrated for scattering of H2O with rare gas atoms. The results show that the ECS approximation, even when only the nearest neighbors are included, is generally much better than the CS approximation in describing scattering. At low collision energies, the ECS gradually converges to the exact results with the increasing number of Coriolis coupled helicity blocks. We further discuss three major factors that may lead to the failure of the CS approximation, namely, the reduced mass, collision energy, and triatomic rotational quantum number. It is illustrated that these factors could impact the relative importance of off-diagonal matrix elements in the Hamiltonian, thus influencing the coupling between different helicity channels.

The integral cross sections and rate constants of pure rotational and ro-vibrational energy transfer processes for the Ar-HF system are thoroughly studied by using the time-independent close coupling method based on our newly constructed potential energy surface. Compared to previous theoretical results, pure rotational transitions in this work achieve better agreement with the experimental data. For ro-vibrational energy transfer, it is found that quasi-resonant transitions dominate the cross sections in all cases. Furthermore, the vibrational-resolved rate constant of transition v=1→v=0 increases very quickly with the temperature from 100 K to 1500 K and is also in good agreement with the available experimental results.

Complex-forming reactions widely exist in gas-phase chemical reactions.Various complexforming bimolecular reactions have been investigated and interesting phenomena have been discovered.The complex-forming reactions usually have small or no barrier in the entrance channel, which leads to obvious differences in kinetic and dynamic characteristics compared with direct reactions.Theoretically, quantum state-resolved reaction dynamics can provide the most detailed microscopic dynamic mechanisms and is now feasible for a direct reaction with only one potential barrier.However, it is of great challenge to construct accurate potential energy surfaces and perform accurate quantum dynamics calculations for a complex polyatomic reaction involving deep potential wells and multi-channels.This paper reviews the most recent progress in two prototypical oxyhydrogen complex-forming reaction systems, HO2 and HO3, which are significant in combustion, atmospheric, and interstellar chemistry.We will present a brief survey of both computational and experimental work and emphasize on some unsolved problems existing in these systems.

We present a full-dimensional ab initio intermolecular potential energy surface (IPES) for the OC-HF van der Waals complex. 3167 ab initio points were computed at the frozen-core (FC) explicitly correlated coupled cluster [FC-CCSD(T)-F12b] level, with the augmented correlation-consistent polarized valence quadruple-zeta basis set plus bond functions. Basis set superposition error correction was also considered by the full counterpoise procedure. Gaussian process regression (GPR) was used to map out the potential energy surface, while a multipole expansion method was employed to smooth the ab initio noise of intermolecular potential in the long range. The global minimum of -1248.364 cm

It is still challenging to accurately qualify the rate coefficients for vibrationally excited molecules in experiment. In particular, for the energy transfer between HF (v = 7) and D2 (v = 0), which is a prototype for near resonant collisional transfer of vibrational excitation from one molecule to the other, the two available experimental results of rate coefficients contradict each other by a factor of nearly 20. In order to benchmark these data, in this work, the rate coefficients of vibration–vibration energy transfer processes of this system at temperatures ranging from 100 to 1500 K were calculated by employing the coupled-states approximation based on our recently developed full-dimensional ab initio intermolecular potential energy surface. The state-to-state rate coefficients were found to follow the general energy gap law. The calculated total vibration–vibration energy transfer rate coefficients decrease with the increase in the angular momentum of HF at most temperatures. The vibrational relaxation rate coefficient decreases monotonously with the temperature, and the calculated result of 8.1 × 10−11 cm3 mol−1 s−1 at room temperature is in very good agreement with the experimental value reported by Dzelzkalns and Kaufman [J. Chem. Phys. 77, 3508 (1982)].

The state-to-state photodissociassion dynamics for the B band of D2O have been explored from quantum dynamical calculations including the electronic X and B states. The calculations were carried out using a Chebyshev real wave packet method. The calculated absorption spectra, product state distributions, and branching ratios from different initial vibrational states show different dynamic features, due to the different shapes of the vibrational wavefunctions. The initial bending mode (0,1,0) generates two lobes with a shallow minimum on the absorption spectrum and a slight inverted vibrational population of OD(X) product at high total energies. The rotational state distributions of OD(X, v=0) product are highly inverted and depend weakly on the initial state and total energy. On the other hand, the ro-vibrational distributions of OD(A) product strongly oscillate with the total energy, which are dominated by the long-living resonances and depend sensitively on the potential surfaces. The antisymmetric...

We present a new full-dimensional ab initio potential energy surface (PES) of a hydrogen fluoride dimer [(HF)2] using the supermolecular approach. The calculations were performed at the coupled-cluster single double triple level, with an augmented correlation-consistent polarized valence quadruple-zeta basis set plus bond functions. The basis set superposition error was corrected by a full counterpoise procedure. With the exchange symmetry of the two HF molecules, the permutation invariant polynomial neural network approach was used to fit the hypersurface with a root-mean-square-error of 0.465 cm-1 for about 110 000 points. The ab initio noise of intermolecular potential in the long range was smoothed by the long-range coefficients method. The equilibrium configuration of the complex was found to be a Cs structure located at two equivalent minima with the well depth of 1573.495 cm-1. The eigenstates were calculated by employing a symmetry-adapted Lanczos propagation algorithm in the mixed radical discrete variable representation/angular finite basis representation. The tunneling splitting for the ground state of (HF)2 is 0.665 cm-1, agreeing well with experimental value of 0.65869 cm-1. Vibrational fundamentals are also very close to the observed values. The results of vibrational states calculations demonstrate the high accuracy of our new PES.

A single set of coordinates, which is optimal for both asymptotic product and reactant, is difficult to find in a state-to-state reactive scattering calculation using the quantum wave packet method. An interaction-asymptotic region decomposition (IARD) method was proposed in this work to solve this "coordinate problem." In the method, the interaction region and asymptotic regions are applied with the local optimal coordinate system, i.e., hyperspherical and corresponding Jacobi coordinates. The IARD method is capable of efficiently and accurately accomplishing a calculation with a grid box for the Jacobi coordinate R extending several hundred bohrs for both reactant and product arrangements. We demonstrate the effectiveness of the IARD method with the reaction of H + HD, which is the simplest direct reaction, and F + HD, which is a typical reaction involving resonances with products of extremely slow translational energy and requires extremely long absorbing potential in all channels.

We report a theoretical study on the rotational spectra of Ar‐D232S. The intermolecular potential energy surface was transformed from our latest ab initio three‐dimensional potential of Ar‐H232S. The rotational energy levels and wavefunctions of the complex were calculated by using the radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm. The calculated rotational transition frequencies and structural parameters were found to be in good agreement with the available experimental values.

The infrared spectra with hydrogen fluoride (HF) and deuterium fluoride (DF) (v2 = 1 ← 0) for eight isotropic species of H2-HF complex are predicted, based on our newly constructed high-accuracy ab initio potential energy surface [D. Yang et al., J. Chem. Phys. 148, 184301 (2018)]. The radial discrete variable representation/angular finite basis representation method and Lanczos algorithm were used to determine the ro-vibrational energy levels and wave functions for eight species of H2-HF complex (para-H2-HF, ortho-H2-HF, para-D2-HF, ortho-D2-HF, para-H2-DF, ortho-H2-DF, para-D2-DF, and ortho-D2-DF) with separating the inter- and intra-molecular vibrations. Bound states properties including their dissociation energies and rotational constants were presented. The calculated band origins are all red shifted to the isolated HF molecule and in good agreement with available experimental values. The frequencies and line intensities of ro-vibrational transitions in the HF stretching band were further calculated, and the predicted infrared spectra are consistent with available observed spectra. Among them, the spectra for three isotopic species of H2-HF (para-H2-DF, para-D2-DF, and ortho-D2-DF) were predicted for the first time.

A full-dimensional ab initio potential energy surface for the H2-HF van der Waals complex was constructed by employing the coupled-cluster singles and doubles with noniterative inclusion of connected triples with augmented correlation-consistent polarised valence quadruple-zeta basis set plus bond functions. Using the improved coupled-states approximation including the nearest neighbor Coriolis couplings, we calculated the state-to-state scattering dynamics for pure rotational and ro-vibrational energy transfer processes. For pure rotational energy transfer, our results showed a different dynamical behavior for para-H2 and ortho-H2 in collision with hydrogen fluoride (HF), which is consistent with the previous study. Interestingly, some strong resonant peaks were presented in the cross sections for ro-vibrational energy transfer. In addition, the calculated vibrational-resolved rate constant is in agreement with the experimental results reported by Bott et al. These dynamics data can be further applied to the numerical simulation of HF chemical lasers.

Solving the time-independent close coupling equations of a diatom-diatom inelastic collision system by using the rigorous close-coupling approach is numerically difficult because of its expensive matrix manipulation. The coupled-states approximation decouples the centrifugal matrix by neglecting the important Coriolis couplings completely. In this work, a new approximation method based on the coupled-states approximation is presented and applied to time-independent quantum dynamic calculations. This approach only considers the most important Coriolis coupling with the nearest neighbors and ignores weaker Coriolis couplings with farther K channels. As a result, it reduces the computational costs without a significant loss of accuracy. Numerical tests for para-H2+ortho-H2 and para-H2+HD inelastic collision were carried out and the results showed that the improved method dramatically reduces the errors due to the neglect of the Coriolis couplings in the coupled-states approximation. This strategy should be useful in quantum dynamics of other systems.

Photodetachment of the ClH2O- anion is investigated using full-dimensional quantum mechanics on accurate potential energy surfaces of both the anion and neutral species. Detailed analysis of the photoelectron spectrum and the corresponding wavefunctions reveals that the photodetachment leads to, in the product channel of the exothermic HCl + OH → Cl + H2O reaction, the formation of numerous Feshbach resonances due apparently to slow energy transfer from H2O vibrational modes to the dissociation coordinate. These long-lived resonances can be grouped into two broad peaks in the low-resolution photoelectron spectrum, which is in good agreement with available experiments, and they are assigned to the ground and first excited OH stretching vibrational manifolds of H2O complexed with Cl. In addition, effects of isotope substitution on the photoelectron spectrum were small. Finally, photodetachment of the vibrationally excited ClH2O- in the ionic hydrogen bond mode is found to lead to Feshbach resonances with higher stretching vibrational excitations in H2O.

New global three dimensional potential energy surfaces for the Cl+H2 reactive system have been constructed using accurate multireference configuration interaction calculations with a large basis set. The three lowest adiabatic potential energy surfaces correlating asymptotically with Cl(2P)+H2 have been transformed to a diabatic representation, which leads to a fourth coupling potential for non-linear geometries. In addition, the spin-orbit coupling surfaces have also been computed using the BreitPauli Hamiltonian. Properties of the new potential are described. Reaction dynamics based on the new potential agrees with the recent experimental results quite well.

Density-functional theory calculations based on plane-wave expansion and pseudopotential treatment were carried out for atomic hydrogen on a rigid Cu(100) surface. A global potential energy surface was then obtained by using a three-dimensional spline interpolation. It is found that the minimum of the potential is located at the fourfold hollow site with a diffusion barrier of 88 meV at the bridge site. The vibrational states of atomic hydrogen and deuterium on the Cu(100) surface were calculated on the potential surface. Our calculations show that the vibrational states A10, A11, E1, and B21 of H/Cu(100) exhibit strong localized character and very narrow band widths, whereas other excited vibrational states have considerable delocalized character and broad band widths. The vibrational frequency of 71.2 (51.5) meV for H(D) in the perpendicular direction obtained in this study is in good agreement with the experimentally observed value of 70 (52) meV [Surf. Sci. 248, 35 (1991)].

A three-dimensional global potential energy surface for the ground electronic state of MgH(2) is constructed from more than 3000 ab initio points calculated using the internally contracted multireference configuration interaction method with the Davidson correction at the complete basis set limit. Low-lying vibrational energy levels of MgH(2) and MgD(2) are calculated using the Lanczos algorithm, and found to be in good agreement with known experimental band origins. The majority of the vibrational energy levels up to 8000 cm(-1) are assigned with normal mode quantum numbers. However, our results indicate a gradual transition from a normal mode regime for the stretching vibrations at low energies to a local mode regime near 7400 cm(-1), as evidenced by a decreasing energy gap between the (n(1),0,0) and (n(1)-1,0,1) vibrational states and bifurcation of the corresponding wave functions.

We report an ab initio intermolecular potential energy surface calculation on the He-N(2)O complex with N(2)O at its ground state using a supermolecular approach. The calculation was performed at the coupled-cluster [CCSD(T)] level, with the full counterpoise correction for the basis set superposition error and a large basis set including midpoint bond functions. The CCSD(T) potential is found to have two minima corresponding to the T-shaped and linear He-ONN structures. The T-shaped minimum is the global minimum. The two-dimensional discrete variable representation method was employed to calculate the rovibrational energy levels for (4)He-N(2)O and (3)He-N(2)O with N(2)O at its ground and nu(3) excited states. The results indicate that the CCSD(T) potential supports five and four vibrational bound states for the (4)He-N(2)O and (3)He-N(2)O, respectively. Moreover, the calculations on the line intensities of the rotational transitions in the nu(3) region of N(2)O for the ground vibrational state shows that the (3)He-N(2)O spectrum is dominated by a-type transitions (DeltaK(a)=0), while the (4)He-N(2)O spectrum is contributed by both the a-type and b-type (DeltaK(a)=+/-1) transitions. The calculated transition frequencies and the intensities are in good agreement with the observed results.

Predissociation dynamics of HCN and DCN in the α absorption band is studied using a newly calculated ab initio potential energy surface of the first excited singlet (1 1A″) state and the corresponding transition dipole surface. The recently proposed complex-symmetric single Lanczos propagation method is applied to generate absorption spectra and fragment internal state distributions for the photodissociation of both HCN and DCN. The absorption spectra of both molecules are dominated by bending progressions, thanks to the linear-to-bent transition. For most low-lying resonances, the CN fragment is found predominantly in its ground vibrational state even when significant energy is available. The absence of fragment vibrational excitation is attributed to the coincidence of the C–N distance at the exit barrier and the dissociation asymptote, and to the lack of final-state interaction outside the barrier. On the other hand, the CN rotational distribution is found to be highly oscillatory and depend on the vib...

The transition-state wave packet method of Zhang and Light [J. Chem. Phys. 104, 6184 (1996)] for the direct calculation of cumulative reaction probabilities is implemented in the Chebyshev order domain to take advantage of exactness and efficiency of the Chebyshev propagator. Numerical testing for three-dimensional H+H2 reactive scattering (J=0) confirms the accuracy and efficiency of the proposed algorithm. This new implementation is then used to compute the cumulative reaction probability of the Li+HF→LiF+H reaction (J=0) up to 0.65 eV. It is found that the latter reaction is dominated in low energy region by numerous narrow resonances.

We report a highly correlated multireference configuration interaction calculation of the near-equilibrium potential energy surface of ozone using a large correlation consistent basis set. Three-dimensional analytical expressions are obtained for the potential energy and dipole moment functions using least-squares fits to ab initio points near the C2v equilibrium geometry. Low-lying vibrational band origins of 16O3 and some of its isotopic variants are calculated using the ab initio potential energy function. The calculated fundamental frequencies for the symmetric stretching and bending vibrations are within about 3 cm−1 of the observed values, while that for the antisymmetric stretch deviates from experiment by about 13 cm−1. The agreement with experiment can be significantly improved if the ab initio potential energy function is scaled in the antisymmetric stretching coordinate. Absolute infrared absorption intensities are also calculated using ab initio electric dipole moment functions and in good agr...

We compare the numerical performance of three recursive methods for calculating collisional resonances, which are characterized by complex eigenenergies of an optical potential augmented Hamiltonian. The first approach involves a modified Chebyshev propagation of a real wave packet, followed by low-storage filter-diagonalization. A similar filter-diagonalization scheme replaces the Chebyshev propagation with a more general Faber recursion associated with a specific conformal mapping in the complex plane. The complex resonance eigenenergies are also obtained using a complex-symmetric version of the Lanczos algorithm. Numerical tests for a realistic triatomic system (HCO) indicate that the Lanczos method and the low-storage filter-diagonalization based on the Chebyshev propagation are much more efficient than the Faber approach.

We report quantum mechanical calculations of the X←C emission spectra of SO2 at the red wing of the C absorption band. The near equilibrium potential energy surface of the C 1B2 state is deduced by fitting experimental vibrational frequencies using an exact quantum mechanical Hamiltonian. Low-lying vibrational eigenenergies on this double minimum potential agree well with experimental frequencies and the corresponding eigenstates show some interesting features. Both spectral positions and intensities of the X←C transitions are obtained up to 16 000 cm−1 using a Chebyshev based spectral method, which does not explicitly construct vibrational eigenfunctions in the X state. The emission spectra are in reasonably good agreement with experimental measurements.

The potential energy surface for the He–CO2 van der Waals complex is calculated using the fourth-order Moller–Plesset (MP4) perturbation theory with a large basis set containing bond functions. The interaction energies are obtained by the supermolecular approach with the full counterpoise correction for the basis set superposition error. The MP4 interaction energies are then fitted to an exponential-spline-Morse–Morse-spline-van der Waals potential form. The ab initio potential energy surface contains two local minima corresponding to the T-shaped and linear structures. The T-shaped minimum is the global minimum with the well depth of 44.41 cm−1 at R(He–C)=3.10 A. The linear minimum has a well depth of 27.69 cm−1 at R(He–C)=4.26 A. A potential barrier that separates the two minima is located at R(He–C)=4.10 A and θ=39° with a height of 19.81 cm−1. The rovibrational energy levels of He–CO2 with CO2 at its ground state and υ3 excited state are calculated using the vibrational self-consistent field configuration-interaction approach. Our calculated results show that the potential supports five vibrational bound states. The zero-point energy is 28.60 cm−1. The first excited bend frequency is 8.67 cm−1, which is very close to the experimental estimated value of about 9±2 cm−1. The calculated transition frequencies and spectroscopic constants are in good agreement with the experimental values.

A global three dimensional potential energy surface for the F+H2HF+H reaction has been developed by spline interpolation of about 15,000 symmetry-unique ab initio points, obtained from the multi-reference configuration interaction level with Davidson correction using the aug-cc-pV5Z basis set. In the entrance channel the spin-orbit coupling energy is also included.

We report a permutationally invariant global potential energy surface (PES) for the H + CH4 system based on ∼63,000 data points calculated at a high ab initio level (UCCSD(T)-F12a/AVTZ) using the recently proposed permutation invariant polynomial-neural network method. The small fitting error (5.1 meV) indicates a faithful representation of the ab initio points over a large configuration space. The rate coefficients calculated on the PES using tunneling corrected transition-state theory and quasi-classical trajectory are found to agree well with the available experimental and previous quantum dynamical results. The calculated total reaction probabilities (Jtot = 0) including the abstraction and exchange channels using the new potential by a reduced dimensional quantum dynamic method are essentially the same as those on the Xu-Chen-Zhang PES [Chin. J. Chem. Phys. 27, 373 (2014)].

The kinetics and dynamics of several O + O2 isotope exchange reactions have been investigated on a recently determined accurate global O3 potential energy surface using a time-dependent wave packet method. The agreement between calculated and measured rate coefficients is significantly improved over previous work. More importantly, the experimentally observed negative temperature dependence of the rate coefficients is for the first time rigorously reproduced theoretically. This negative temperature dependence can be attributed to the absence in the new potential energy surface of a submerged "reef" structure, which was present in all previous potential energy surfaces. In addition, contributions of rotational excited states of the diatomic reactant further accentuate the negative temperature dependence.

The reactions between N((4)S) and CH(X(2)Π) lead to H((2)S) plus CN in its two lowest electronic states (X(2)Σ(+) and A(2)Π), which are responsible for the interstellar CN formation. Accurate quantum dynamics of these reactions are investigated on new global potential energy surfaces of the two lowest-lying triplet states of HCN (1(3)A' and 1(3)A") fitted to more than 37,000 points at the internally contracted multi-reference configuration interaction level with the Davidson correction. The pathways for these highly exothermic and barrierless reactions feature both the HCN and HNC wells. Long-lived resonances supported by these wells manifest in reaction probabilities as numerous oscillations, particularly for low J partial waves. The 1(3)A" state is found to be more reactive than the 1(3)A' state, due apparently to its more attractive nature in the entrance channel. The CN products in both electronic states are highly excited in both vibrational and rotational degrees of freedom. The near forward-backward symmetric differential cross sections are consistent with a complex-forming mechanism.

The adiabatic state-to-state dynamics of the reaction between O((3)P) and NH(X(3)Σ(-)) has been investigated on three lowest-lying electronic states, namely, the X(1)A('), A(1)A("), and a(3)A(") states, using the recently developed global potential energy surfaces based on high level ab initio data. The reaction rate has contributions from all three states, with the largest coming from the triplet state. The rotational and vibrational degrees of freedom of the prominent NO product are highly excited, although significant differences exist in the internal state distributions of the three adiabatic channels. The reaction proceeds with a complex-forming mechanism on all three electronic states, as evidenced by resonance structures in reaction probabilities and the near forward-backward symmetry in the differential cross section. However, significant non-reactive scattering and inverted vibrational state distributions suggest substantial non-statistical behaviors.

We report a new three-dimensional ab initio intermolecular potential energy surface for the Kr-H(2)O complex with the H(2)O monomer fixed at its experimental averaged structure. Using the supermolecular approach, the intermolecular potential energies were evaluated at the coupled-cluster singles and doubles level with noniterative inclusion of connected triples with the full counterpoise correction for the basis set superposition error and a large basis set including bond functions. The global minimum corresponding to a planar H-bond configuration was located at the intermolecular distance of 3.82 Å with a well depth of 169.98 cm(-1). In addition, two first-order and one second-order saddle points were also identified. The combined radial discrete variable representation∕angular finite basis representation method and the Lanczos algorithm were employed to calculate the rovibrational energy levels for 16 isotopic species of the Kr-H(2)O complexes. The rotational transition frequencies, structure parameters, and nuclear quadrupole coupling constants were also determined for the ground and first intermolecular vibrational excited states and are all in good agreement with the available experimental values.

We report finite temperature quantum mechanical simulations of structural and dynamical properties of Ar(N)-CO(2) clusters using a path integral Monte Carlo algorithm. The simulations are based on a newly developed analytical Ar-CO(2) interaction potential obtained by fitting ab initio results to an anisotropic two-dimensional Morse/Long-range function. The calculated distributions of argon atoms around the CO(2) molecule in Ar(N)-CO(2) clusters with different sizes are consistent to the previous studies of the configurations of the clusters. A first-order perturbation theory is used to quantitatively predict the CO(2) vibrational frequency shift in different clusters. The first-solvation shell is completed at N = 17. Interestingly, our simulations for larger Ar(N)-CO(2) clusters showed several different structures of the argon shell around the doped CO(2) molecule. The observed two distinct peaks (2338.8 and 2344.5 cm(-1)) in the υ(3) band of CO(2) may be due to the different arrangements of argon atoms around the dopant molecule.

A detailed quasi-classical trajectory study of the H + CO(2) → HO + CO reaction is reported on an accurate potential energy surface based on ab initio data. The influence of the vibrational and rotational excitations of CO(2) was investigated up to the collision energy of 2.35 eV. It was found that the total reaction integral cross section increases monotonically with the collision energy, consistent with experimental results. The excitation of the CO(2) bending vibration enhances the reaction, while the excitation in its asymmetric stretching vibration inhibits the reaction. The calculated thermal rate constants are in excellent agreement with experiment. At the state-to-state level, the rotational state distributions of the HO product are in good agreement with experimental results, while those for the CO product are much hotter than measurements. The calculated differential cross sections are dominated by forward scattering, suggesting that the lifetime of the HOCO intermediate may not be sufficiently long to render the reaction completely statistical.

We report a reliable three-dimensional ab initio intermolecular potential energy surface for the Ar-H(2)S complex with H(2)S monomer fixed at its experimental average structure. The potential energies were evaluated using the supermolecular approach at the coupled-cluster level with a large basis set including bond functions. The full counterpoise procedure was used to correct the basis set superposition error. The potential has a planar T-shaped global minimum with a well depth of 177.48 cm(-1) at the intermolecular distance of 3.72 Å. An additional planar local minimum is also found and is separated from the global minimum with an energy barrier with a height of 47.46 cm(-1). The combined radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm were employed to calculate the rovibrational energy levels for three isotopic species of Ar-H(2)S complexes (Ar-H(2)(32)S, Ar-H(2)(33)S, and Ar-H(2)(34)S). The rotational transition frequencies and structural parameters for the three isotopomers were also determined for the ground and the first excited states, which are all in good agreement with the available experimental values.

We report a chemically accurate global potential energy surface for the HOCO system based on high-level ab initio calculations at ~35,000 points. The potential energy surface is shown to reproduce important stationary points and minimum energy paths. Quasi-classical trajectory calculations indicated a good agreement with experimental data.

The three lowest (1A('), 2A('), and 1A('')) adiabatic potential energy surfaces (PESs) for the Br((2)P) + H(2) reactive system have been computed based on the multi-reference configuration interaction (MRCI) method including the Davidson's correction with a large basis set. These three adiabatic PESs have been transformed to a diabatic representation, leading to four coupling potentials. In addition, the spin-orbit matrix elements were also obtained using the Breit-Pauli Hamiltonian and the unperturbed MRCI wavefunctions in the Br + H(2) channel and the transition state region. Consequently, six coupling potentials were obtained and their characteristics were extensively discussed. Nonadiabatic quantum dynamics calculations for this system have been realized with these realistic diabatic potentials instead of previous semi-empirical diabatic potentials. Based on two-state model nonadiabatic calculations for the Br((2)P(3∕2), (2)P(1∕2)) + H(2) reaction, the Br((2)P(1∕2)) + H(2) reaction was found to show less reactivity than the Br((2)P(3∕2)) + H(2) reaction at collision energies beyond the threshold of the Br((2)P(3∕2)) + H(2) reaction. Our results are consistent with the previous studies on the XH(2) (X = F, Cl) system, which indicate that the adiabatically forbidden channel is dominant at low energies in the open-shell halogen atom plus H(2) reactions.

A Chebyshev-based flux-flux correlation function approach is introduced for calculating multiple initial state selected reaction probabilities for bimolecular reactions. Based on the quantum transition-state theory, this approach propagates, with the exact Chebyshev propagator, transition-state wave packets towards the reactant asymptote. It is accurate and efficient if many initial state selected reaction probabilities are needed. This approach is applied to the title reaction to elucidate the influence of the H(2)O ro-vibrational states on its reactivity. Results from several potential energy surfaces are compared.

Quantum state-to-state differential cross sections, along with the absorption spectrum and product internal state distributions, have been calculated for the photodissociation of H(2)O in its B band on a new set of ab initio potential energy surfaces in a diabatic representation. The theoretical attributes are in good agreement with the recent experimental data, shedding light on the non-adiabatic dissociation dynamics.

A new global potential energy surface for the lowest triplet electronic state (a(3)A") of HNO has been developed by a three-dimensional cubic spline interpolation of more than 13,000 ab initio points, which were calculated at the multireference configuration interaction level with Davidson correction using the augmented correlation-consistent polarized valence quintuple zeta basis set. Two minima and five saddle points were found on the potential energy surface. Low-lying vibrational states were obtained in this new potential using the Lanczos method and assigned. In addition, thermal rate constants for the N + OH → H + NO reactions were obtained using an exact wave packet method. Reasonably good agreement with experimental data was obtained.

Quantum state-to-state dynamics for the H + HBr(υ(i) = 0, j(i) =0) reaction was studied on an accurate ab intio potential energy surface for the electronic ground state of BrH(2). Both the H + HBr → H(2) + Br abstraction reaction and the H' + HBr → H'Br + H exchange reaction were investigated up to a collision energy of 2.0 eV. It was found that the abstraction channel is dominant at lower collision energies, while the exchange channel becomes dominant at higher collision energies. The total integral cross section of the abstraction reaction at a collision energy of 1.6 eV was found to be 1.37 Å(2), which is larger than a recent quantum mechanical result (1.06 Å(2)) and still significantly smaller than the experimental value (3 ± 1 Å(2)). Meanwhile, similar to the previous theoretical study, our calculations also predicted much hotter product rotational state distributions than those from the experimental study. This suggests that further experimental investigations are highly desirable to elucidate the dynamic properties of the title reactions.

The authors report extensive high-level ab initio studies of the first excited (Ã (2)A(')) state of HO(2). A global potential energy surface (PES) was developed by spline-fitting 17 000 ab initio points at the internal contracted multireference configuration interaction (icMRCI) level with the AVQZ basis set. To ascertain the spectroscopic accuracy of the PES, the near-equilibrium region of the molecule was also investigated using three interpolating moving least-squares-based PESs employing dynamically weighted icMRCI methods in the complete basis set limit. Vibrational energy levels on all four surfaces agree well with each other and a new assignment of some vibrational features is proposed. In addition, the dynamics of both the forward and reverse directions of the H+O(2)(ã (1)Δ(g))↔OH+O reaction (J=0) were studied using an exact wave packet method. The reactions are found to be dominated by sharp resonances.

We report a new three-dimensional potential energy surface for Ne-CO(2) including the Q(3) normal mode for the υ(3) antisymmetric stretching vibration of the CO(2) molecule. The potential energies were calculated using the supermolecular method at the coupled-cluster singles and doubles level with noniterative inclusion of connected triples [CCSD(T)], using a large basis set supplemented with midpoint bond functions. Two vibrationally averaged potentials with CO(2) at both the ground (υ=0) and the first (υ=1) vibrational υ(3) excited states were generated from the integration of the three-dimensional potential over the Q(3) coordinate. Each potential was found to have a T-shaped global minimum and two equivalent linear local minima. The radial DVR/angular FBR method and the Lanczos algorithm are applied to calculate the rovibrational energy levels. Comparison with the available observed values showed an overall excellent agreement for the microwave and infrared spectra. The calculated band origin shifts were found to be 0.1306 and 0.1419 cm(-1) for Ne-CO(2) and Ne-C(18)O(2), respectively, which are very close to the experimental values of 0.1303 and 0.1432 cm(-1).

The authors report a detailed quantum mechanical study of the state-to-state dynamics of the O +OH(nu(i)=0, j(i)=0)-> H+O-2(nu(f), j(f)) reaction on an accurate HO2(X(2)A '') potential energy surface. The scattering dynamics was treated using a reactant coordinate based Chebyshev real wavepacket method with full Coriolis coupling. A total of 84 partial waves were calculated in order to achieve convergence up to the collision energy of 0.17 eV. The differential cross section is near forward-backward symmetric, consistent with the complex-forming mechanism. The O-2 product was found to have a monotonically decaying vibrational distribution and highly excited and inverted rotational distributions, also consistent with the formation of the HO2 intermediate. These quantum mechanical results were compared with those obtained in earlier quasiclassical trajectory and statistical studies and it is shown that the statistical theory gives a reasonably good description of the product state distributions despite its inability to predict the total reaction cross section. (C) 2010 American Institute of Physics. [doi:10.1063/1.3455431]

Quantum state-to-state dynamics for the NH + H/D --H + NH/ND exchange reaction is reported on an accurate ab initio potential energy surface. The differential cross section is dominated by scattering into both the forward and backward directions, while the product shows strong rotational excitation. The calculated rate constant for the NH + D reaction is in excellent agreement with experiment. The applicability of a quantum statistical model is tested against exact state-to-state quantum results. Due to strong nonreactive scattering, the statistical model significantly overestimates the integral cross section. However, it is shown that the product state distribution and differential cross sections are reasonably well reproduced by the statistical model because the reactive scattering is dominated by a complex-forming mechanism.

A global potential-energy surface for the first excited electronic state of NH(2)(A(2)A(')) has been constructed by three-dimensional cubic spline interpolation of more than 20,000 ab initio points, which were calculated at the multireference configuration-interaction level with the Davidson correction using the augmented correlation-consistent polarized valence quadruple-zeta basis set. The (J=0) vibrational energy levels for the ground (X(2)A(")) and excited (A(2)A(')) electronic states of NH(2) were calculated on our potential-energy surfaces with the diagonal Renner-Teller terms. The results show a good agreement with the experimental vibrational frequencies of NH(2) and its isotopomers.

We report global potential energy surfaces for both the ground (X1A′) and the excited (A1A′′) electronic states of HGeBr as well as the transition dipole moment surface between them using an internally contracted multireference configuration interaction method with the Davidson correction and an augmented correlation-consistent polarized valence quadruple-ζ basis set. Vibrational energy levels of HGeBr and DGeBr are calculated on both the ground and the excited electronic states and found in good agreement with the available experimental band origins. In addition, the A1A′′−X1A′ absorption and emission spectra of the two isotopomers were obtained, and an excellent agreement with the available experimental spectra was found.