A procedure for the transformation from reactant to product Jacobi coordinates is proposed, which is designed for the extraction of state-to-state reaction probabilities using a time-dependent method in a body-fixed frame. The method consists of several steps which involve a negligible extra computational time as compared with the propagation. Several intermediate coordinates are used, in which the efficiency depends on the masses of the atoms involved in the reaction. A detailed study of the relative efficiency of using reactant and product Jacobi coordinates is presented for several systems, and simple arguments are found depending on the masses of the atoms involved in the reaction. It is found that the proposed method is, in general, more efficient than the use of product Jacobi coordinates, specially for nonzero total angular momentum. State-to-state reaction probabilities are obtained for Li+FH-->LiF+H and F+HO-->FH+O collisions for several total angular momenta.

The state-to-state differential cross sections for some atom + diatom reactions have been calculated using a new wave packet code, MAD-WAVE3, which is described in some detail and uses either reactant or product Jacobi coordinates along the propagation. In order to show the accuracy and efficiency of the coordinate transformation required when using reactant Jacobi coordinates, as recently proposed [ J. Chem. Phys. 2006 , 125 , 054102 ], the method is first applied to the H + D(2) reaction as a benchmark, for which exact time-independent calculations are also performed. It is found that the use of reactant coordinates yields accurate results, with a computational effort slightly lower than that when using product coordinates. The H(+) + D(2) reaction, with the same masses but a much deeper insertion well, is also studied and exhibits a completely different mechanism, a complex-forming one which can be treated by statistical methods. Due to the longer range of the potential, product Jacobi coordinates are more efficient in this case. Differential cross sections for individual final rotational states of the products are obtained based on exact dynamical calculations for some selected total angular momenta, combined with the random phase approximation to save the high computational time required to calculate all partial waves with very long propagations. The results obtained are in excellent agreement with available exact time-independent calculations. Finally, the method is applied to the Li + HF system for which reactant coordinates are very well suited, and quantum differential cross sections are not available. The results are compared with recent quasiclassical simulations and experimental results [J. Chem. Phys. 2005, 122, 244304]. Furthermore, the polarization of the product angular momenta is also analyzed as a function of the scattering angle.

The rate constants required to model the OH + observations in different regions of the interstellar medium have been determined using state of the art quantum methods. First, state-to-state rate constants for the H 2 (v = 0, J = 0, 1)+ O + ( 4 S) → H + OH + (X 3 Σ − , v ′ , N ) reaction have been obtained using a quantum wave packet method. The calculations have been compared with time-independent results to asses the accuracy of reaction probabilities at collision energies of about 1 meV. The good agreement between the simulations and the existing experimental cross sections in the 0.01−1 eV energy range shows the quality of the results. The calculated state-to-state rate constants have been fitted to an analytical form. Second, the Einstein coefficients of OH + have been obtained for all astronomically significant ro-vibrational bands involving the X 3 Σ − and/or A 3 Π electronic states. For this purpose the potential energy curves and electric dipole transition moments for seven electronic states of OH + are calculated with ab initio methods at the highest level and including spin-orbit terms, and the rovibrational levels have been calculated including the empirical spin-rotation and spin-spin terms. Third, the state-to-state rate constants for inelastic collisions between He and OH + (X 3 Σ − ) have been calculated using a time-independent close coupling method on a new potential energy surface. All these rates have been implemented in detailed chemical and radiative transfer models. Applications of these models to various astronomical sources show that inelastic collisions dominate the excitation of the rotational levels of OH + . In the models considered the excitation resulting from the chemical formation of OH + increases the line fluxes by about 10 % or less depending on the density of the gas.

A theoretical study of the F(2P) + OH(2Pi) --> HF(1Sigma+) + O(3P) reactive collisions is carried out on a new global potential energy surface (PES) of the ground 3A" adiabatic electronic state. The ab initio calculations are based on multireference configuration interaction calculations, using the aug-cc-pVTZ extended basis sets of Dunning et al. A functional representation of the PES shows no nominal barrier to reaction, contrary to previous results by others. Wave packet and quasiclassical trajectory calculations have been performed for this PES to study the F + OH(v = 0,j) reactive collision. The comparison was performed at fixed and constant values of the total angular momentum from 0 to 110 and relative translational energy up to 0.8 eV. The reaction presents a dynamical barrier, essentially due to the zero-point energy for the bending vibration near the saddle point. This determines two different reaction mechanisms. At energies higher than approximately 0.125 eV the reaction is direct, while below that value it is indirect and mediated by heavy-light-heavy resonances. Such resonances, also found in the simulations of the photodetachment spectrum of the triatomic anion, manifest themselves in the quasiclassical simulations, too, where they are associated to periodic orbits.

In this work we present a dynamically biased statistical model to describe the evolution of the title reaction from statistical to a more direct mechanism, using quasi-classical trajectories (QCT). The method is based on the one previously proposed by Park and Light [J. Chem. Phys. 126, 044305 (2007)]. A recent global potential energy surface is used here to calculate the capture probabilities, instead of the long-range ion-induced dipole interactions. The dynamical constraints are introduced by considering a scrambling matrix which depends on energy and determine the probability of the identity/hop/exchange mechanisms. These probabilities are calculated using QCT. It is found that the high zero-point energy of the fragments is transferred to the rest of the degrees of freedom, what shortens the lifetime of H + 5 complexes and, as a consequence, the exchange mechanism is produced with lower proportion. The zero-point energy (ZPE) is not properly described in quasiclassical trajectory calculations and an approximation is done in which the initial ZPE of the reactants is reduced in QCT calculations to obtain a new ZPE-biased scrambling matrix. This reduction of the ZPE is explained by the need of correcting the pure classical level number of the H + 5 complex, as done in classical simulations of unimolecular processes and to get equivalent quantum and classical rate constants using Rice-Ramsperger-Kassel-Marcus theory. This matrix allows to obtain a ratio of hop/exchange mechanisms, α(T), in rather good agreement with recent experimental results by Crabtree et al. [J. Chem. Phys. 134, 194311 (2011)] at room temperature. At lower temperatures, however, the present simulations predict too high ratios because the biased scrambling matrix is not statistical enough. This demonstrates the importance of applying quantum methods to simulate this reaction at the low temperatures of astrophysical interest.

Global three-dimensional adiabatic potential-energy surfaces for the excited 2(3)A" and 1(3)A' triplet states of OHF are obtained to study the F(2P)+OH(2pi)-->O(3P)+HF(1sigma+) reaction. Highly accurate ab initio calculations are obtained for the two excited electronic states and fitted to analytical functions with small deviations. The reaction dynamics is studied using a wave-packet treatment within a centrifugal sudden approach, which is justified by the linear transition state of the two electronic states studied. The reaction efficiency presents a marked preference for perpendicular orientation of the initial relative velocity vector and the angular momentum of the OH reagent, consistent in the body-fixed frame used with an initial collinear geometry which facilitates the access to the transition state. It is also found that the reaction cross section presents a rather high threshold so that, in an adiabatic picture, the two excited triplet states do not contribute to the rate constant at room temperature. Thus, only the lowest triplet state leads to reaction under these conditions and the simulated rate constants are too low as compared with the experimental ones. Such disagreement is likely to be due to nonadiabatic transitions occurring at the conical intersections near the transition state for this reaction.

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The dynamics and kinetics of the LiH + H reaction have been studied by using an accurate quantum reactive time-dependent wave packet method on the ab initio ground electronic state potential energy surfaces (PES) developed earlier. Reaction probabilities for the two possible reaction channels, the LiH + H→ H 2 + Li depletion process and the LiH + H→H + LiH hydrogen exchange reaction, have been calculated from 1 meV up to 1.0 eV collision energies for total angular momenta J from 0 to 80. State-to-state and total integral cross sections for the LiH-depletion and H-exchange channels of the reaction have been calculated over this collision energy range. It is found that the LiH-depletion channel is dominant in the whole range of collision energies for both PESs. Accurate total rate coefficients have been calculated on both surfaces from 100 K to 2000 K and are significantly larger than previous empirical estimates and previous J-shifting results. In addition, the present accurate calculations present noticeable differences with previous calculations using the centrifugal sudden approximation.