Quasiclassical Trajectory Study of the Si + SH Reaction on an Accurate Double Many-Body Expansion Potential Energy Surface.

An accurate potential energy surface (PES) for the HSiS system based on MRCI+Q calculations extrapolated to the complete basis set limit is presented. Modeled with the double many-body expansion (DMBE) method, the PES provides an accurate description of the long-range interactions, including electrostatic and dispersion terms decaying as R-4, R-5, R-6, R-8, R-10 that are predicted from dipole moments, quadrupole moments, and dipolar polarizabilities, which are also calculated at the MRCI+Q level. The novel PES is then used in quasiclassical trajectory calculations to predict the rate coefficients of the Si + SH → SiS + H reaction, which has been shown to be a major source of the SiS in certain regions of the interstellar medium. An account of the zero-point energy leakage based on various nonactive models is also given. It is shown that the reaction is dominated by long-range forces, with the mechanism Si + SH → SiSH → SSiH → SiS + H being the most important one for all temperatures studied. Although SSiH corresponds to the global minimum of the PES, the contribution from the direct reaction Si + SH → SSiH → SiS + H is less than 0.5% for temperatures higher than 500 K. The rovibrational distributions of the products are calculated using the momentum Gaussian binning method and show that as the temperature is increased the average vibrational quantum number decreases while the rotational distribution spreads up to larger values.

The O + NO( v) Vibrational Relaxation Processes Revisited.

We have carried out a quasiclassical trajectory study of the O + NO( v) energy transfer process using DMBE potential energy surfaces for the ground-states of the 2A' and 2A″ manifolds. State-to-state vibrational relaxation rate constants have been computed over the temperature range 298 and 3000 K and initial vibrational states between v = 1 and 9. The momentum-Gaussian binning approach has been employed to calculate the probability of the vibrational transitions. A comparison of the calculated state-to-state rate coefficients with the results from experimental studies and previous theoretical calculations shows the relevance of the 1 2A″ potential energy surface to the title vibrational relaxation process.

Dynamics of the O + ClO reaction: reactive and vibrational relaxation processes.

Classical trajectories have been integrated to study the O + ClO reaction, both reactive and vibrational energy transfer processes, for the range of temperatures 100 ≤ T/K ≤ 500 using momentum Gaussian binning. The employed potential energy surface is the recently proposed single-sheeted double many-body expansion potential energy surface for the (2)A" ground-state of ClO2 based on multireference ab initio data. A capture-type regime with a room-temperature rate constant of (17.8 ± 0.5) × 10(-12) cm(3) s(-1) and temperature dependence of k(T/K)/cm(3) s(-1) = 22.4 × 10(-12) × T(-0.81) exp(-39.2/T) has been found. Although the value reported here is half of the experimental and recommended one, tentative explanations are given. Other dynamical attributes are also examined for the title reaction, with state-to-all and state-to-state vibrational relaxation and excitation rate constants reported for temperatures of relevance in stratospheric chemistry.

N(4S ∕2D)+N2: accurate ab initio-based DMBE potential energy surfaces and surface-hopping dynamics.

This work gives a full account of the N((4)S∕(2)D)+N(2)((1)Σ(g)(+)) interactions via accurate electronic structure calculations and study of the involved exchange reactions. A 2 × 2 diabatic representation of the potential energy surface is suggested for N(3)((2)A(')), which, combined with the two previously reported adiabatic forms for (2)A(") and another for (4)A("), completes the set of five global potentials required to study the title collisional processes. The trajectory results provide the first N((2)D)+N(2) rate constants, and allow a comparison with the ones for N((4)S)+N(2). Nonadiabatic effects are estimated by surface hopping, and the geometrical phase effect assessed by following the trajectories that encircle the crossing seam.

Ab initio-based global double many-body expansion potential energy surface for the first 2A" electronic state of NO2.

An ab initio-based global double many-body expansion potential energy surface is reported for the first electronic state of the NO(2)((2)A") manifold. Up to 1700 ab initio energies have been employed to map the full configuration space of the title molecule, including stationary points and asymptotic channels. The calculated grid energies have been scaled to account for the incompleteness of the basis set and truncation of the MRCI expansion and fitted analytically with a total root-mean-squared-deviation smaller than 1.0 kcal mol(-1). The lowest point of the potential energy surface corresponds to the (2)B(1) linear minimum, which is separated from the C(s) distorted minimum by a C(2v) barrier of ≈9.7 kcal mol(-1). As usual, the proposed form includes a realistic representation of long-range interactions. Preliminary work indicates its reliability for reaction dynamics.

Quasiclassical trajectory study of the C(1D) + H2 reaction and isotopomeric variants: kinetic isotope effect and CD/CH branching ratio.

The recently proposed ab initio single-sheeted double many-body expansion potential energy for the methylene molecule has been used to perform quasiclassical trajectory (QCT) calculations for the title reaction. Thermal and initial state-specific (v = 0, j = 0) rate constants for the C((1)D) + H(2)/HD/D(2) reactions have been obtained over a wide range of temperatures. Cross sections for the reaction C((1)D) + H(2) and its deuterated isotopes have also been calculated, as well as the CD/CH branching ratios for the C((1)D) + HD reaction. It is found that the CD + H product channel in the C((1)D) + HD reaction is preferred relative to the CH + D channel. The estimated rate constants are predicted to be in the order k(H2) > k(HD) > k(D2) and the calculated cross sections and rate constants compared with available theoretical and experimental data.

Quasiclassical trajectory study of atom-exchange and vibrational relaxation processes in collisions of atomic and molecular nitrogen.

Quasiclassical trajectories have been integrated to study the exchange reaction of molecular nitrogen in collisions with atomic nitrogen for temperatures over the range of 1273 < or = T (K) < or = 10,000. A recently proposed potential energy surface for the ground A'' quartet state of the system has been employed. If compared to previous theoretical studies, the results of the present work show a higher reactivity due to a lower barrier, with a study of the effect of this height in the thermal rate constant being also performed. Vibrational energy transfer via chemical reaction and/or inelastic collisions are also studied.

Potential energy surface for ground-state H2S via scaling of the external correlation, comparison with extrapolation to complete basis set limit, and use in reaction dynamics.

A global double many-body expansion potential energy surface is reported for the electronic ground state of H2S by fitting accurate ab initio energies calculated at the multireference configuration interaction level with the aug-cc-pVQZ basis set, after slightly correcting semiempirically the dynamical correlation by the double many-body expansion-scaled external correlation method. The function so obtained has been compared in detail with a potential energy surface of the same type recently reported (Song , Y. Z. and Varandas , A. J. C. J. Chem. Phys. 2009 , 130 , 134317.) by extrapolating the calculated raw energies to the complete basis set limit, eschewing any use of information alien to ab initio theory. The new potential energy surface is also used for studying the dynamics and kinetics of the S(1D) + H2/D2/HD reactions.

A theoretical study of rate coefficients for the O + NO vibrational relaxation.

Quasi-classical trajectories have been integrated to study the vibrational relaxation of the O + NO(v) process as a function of the initial vibrational quantum number for T = 298 K, 1500 K, and 3000 K. Two reliable potential energy surfaces have been employed for the A' and A'' doublet states of NO2. The calculated vibrational relaxation rate constants show a nearly v-independent behavior at room temperature and a moderate increase with v for higher temperatures. Although deviating significantly from the recommended values, good agreement with recent experimental results has been obtained. The importance of multi-quantum transitions is also analyzed.

Recalibrated double many-body expansion potential energy surface and dynamics calculations for HN2.

A single-sheeted double many-body expansion potential energy surface is reported for the lowest doublet state of HN2 by fitting additional multireference configuration interaction energies in the N...NH channel. A stratified analysis of the root-mean-squared error indicates an accuracy superior to that achieved for the previously reported form. Detailed dynamical tests are also performed for the N + NH reaction using both the quasi-classical trajectory method and the capture theory, and the results are compared with available empirical data. The vibrational resonances of the HN2 metastable radical are also calculated and compared with previous theoretical predictions.

Dynamics of X+CH4 (X=H,O,Cl) reactions: how reliable is transition state theory for fine-tuning potential energy surfaces?

Trajectory calculations run on global potential energy surfaces have shown that the topology of the entrance channel has strong implications on the dynamics of the title reactions. This may explain why huge differences are observed between the rate constants calculated from global dynamical methods and those obtained from local methods that employ the same potential energy surfaces but ignore such topological details. Local dynamics approaches such as transition state-based theories should then be used with caution for fine-tuning potential energy surfaces, especially for fast reactions with polyatomic species since the key statistical assumptions of the theory may not be valid for all degrees of freedom.

Unimolecular and bimolecular calculations for HN2.

Using a recently reported double many-body expansion potential energy surface, quasi-classical, statistical mechanics, and quantum resonance calculations have been performed for the HN(2) system by focusing on the determination of bimolecular (N + NH and H + N(2)) and unimolecular (decomposition of HN(2)) rate constants as well as the relevant equilibrium constants.