Miguel Paniagua

A new functional form to obtain analytical potentials of triatomic molecules

A new way of fitting ab initio values of the potential energy of triatomic molecules is discussed. The new functional form proposed for the potential energy function satisfies several criteria for use in scattering calculations and it is so stable that, when increasing the number of parameters in the fit, it maintains a good behavior at short, intermediate, and long range of the whole potential. Applications of the fitting technique to obtain the potential energy functions for the ground states of H3, H2He+, and LiHF are presented. For the H3 system, we fit all 267 of Liu and Siegbahn’s ab initio points. With one nonlinear parameter and 23 linear parameters, the root‐mean‐square error was 1.21 kcal/mol and the maximum absolute deviation 7.94 kcal/mol; these values are lower than previous global fittings with a similar number of parameters except for the Truhlar and Horowitz fit of the Liu and Siegbahn data, LSTH potential, but when the number of linear parameters was increased to 71, the root‐mean‐square ...

Global potential energy surfaces for the H3+ system. Analytical representation of the adiabatic ground-state 1 1A′ potential

Adiabatic global potential energy surfaces, for singlet and triplet states of A′ and A″ symmetries, were computed for an extensive grid for a total of 8469 conformations of H3+ system at full configuration interaction ab initio level and using an extended basis set that has also been optimized for excited states. An accurate (root-mean-square error lower than 20 cm−1) global fit to the ground-state potential is obtained using a diatomics-in-molecules approach corrected by several symmetrized three-body terms with a total of 96 linear parameters and 3 nonlinear parameters. This produces an accurate global potential which represents all aspects of ground-state H3+ including the absolute minimum, the avoided crossing and dissociation limits, satisfying the correct symmetry properties of the system. The rovibrational eigenstates have been calculated up to total angular momentum J=20 using hyperspherical coordinates with symmetry adapted basis functions. The infrared spectra thus reproduced is within 1 cm−1 wi...

Quantum study of the Li+HF→LiF+H reaction

In this work we present a new global fit for the potential energy surface of the LiFH system. This fit is an improvement of a recently published one [Aguado et al., J. Chem. Phys. 106, 1013 (1997)] for which more ab initio points have been calculated (from 644 to 2323). The reaction dynamics is studied using a time dependent treatment in reactant Jacobi coordinates in a body-fixed frame in which the internal coordinates are represented on a grid while Eulerian angles are described in a basis set. The centrifugal sudden approach is tested for total angular momentum J=5 and used to calculate the reaction cross section. The reaction cross section shows oscillations as a function of kinetic energy. This is a consequence of strong interference effects between reactant and product channels and is in agreement with the recent experimental data.

The H3+ rovibrational spectrum revisited with a global electronic potential energy surface

In this paper, we have computed the rovibrational spectrum of the H(3) (+) molecule using a new global potential energy surface, invariant under all permutations of the nuclei, that includes the long range electrostatic interactions analytically. The energy levels are obtained by a variational calculation using hyperspherical coordinates. From the comparison with available experimental results for low lying levels, we conclude that our accuracy is of the order of 0.1 cm(-1) for states localized in the vicinity of equilateral triangular configurations of the nuclei, and changes to the order of 1 cm(-1) when the system is distorted away from equilateral configurations. Full rovibrational spectra up to the H(+)+H(2) dissociation energy limit have been computed. The statistical properties of this spectrum (nearest neighbor distribution and spectral rigidity) show the quantum signature of classical chaos and are consistent with random matrix theory. On the other hand, the correlation function, even when convoluted with a smoothing function, exhibits oscillations which are not described by random matrix theory. We discuss a possible similarity between these oscillations and the ones observed experimentally.

Accurate global fit of the H4 potential energy surface

A global potential energy surface (PES) for the adiabatic ground state of the H4 system which fit published ab initio data [Boothroyd, et al. J. Chem. Phys. 95, 4331 (1991)] at a quantitative level has been obtained (root‐mean‐square error about 2 mhartree or 1–2 kcal/mol), and without any quantity of ad hoc character, preserving the accuracy of the ab initio points. The global fitting procedure used here is an extension of the corresponding procedure for triatomic systems including the functional form previously proposed by the authors. The global H4 PES obtained here is totally symmetric with respect to permutations of the hydrogen atoms and satisfies the criteria needed to be used in scattering calculations.

Quantum stereodynamics of the Li HF v,j reactive collision for different initial states of the reagent

The effect of the reagent initial state excitation on the reactive cross section in the Li+HF(v,j) collision is analyzed for v=0, 1 and j=0, 1, 2 and 3. A wave packet treatment is used within the centrifugal sudden approximation on a global potential energy surface recently proposed [Aguado et al., J. Chem. Phys. 107, 10085 (1997)]. The reaction cross-section for v=0 is in good agreement with the available experimental data, and for low j shows oscillations as a function of the translational energy which are due to the structure of the transition state. For v=1 the reaction cross-section increases by a factor of 10–50 with respect to that of v=0. The influence of the alignment of the initial angular momentum on the reaction cross section is studied.

Quantum approaches for the insertion dynamics of the H++D2 and D++H2 reactive collisions

The H(+)+D(2) and D(+)+H(2) reactive collisions are studied using a recently proposed adiabatic potential energy surface of spectroscopic accuracy. The dynamics is studied using an exact wave packet method on the adiabatic surface at energies below the curve crossing occurring at approximately 1.5 eV above the threshold. It is found that the reaction is very well described by a statistical quantum method for a zero total angular momentum (J) as compared with the exact ones, while for higher J some discrepancies are found. For J >0 different centrifugal sudden approximations are proposed and compared with the exact and statistical quantum treatments. The usual centrifugal sudden approach fails by considering too high reaction barriers and too low reaction probabilities. A new statistically modified centrifugal sudden approach is considered which corrects these two failures to a rather good extent. It is also found that an adiabatic approximation for the helicities provides results in very good agreement with the statistical method, placing the reaction barrier properly. However, both statistical and adiabatic centrifugal treatments overestimate the reaction probabilities. The reaction cross sections thus obtained with the new approaches are in rather good agreement with the exact results. In spite of these deficiencies, the quantum statistical method is well adapted for describing the insertion dynamics, and it is then used to evaluate the differential cross sections.

State-to-state reaction probabilities using bond coordinates: Application to the Li+HF(v,j) collision

In this work we present a time-dependent method to evaluate state-to-state reaction probabilities, based on bond coordinates and an adapted body-fixed frame. Such a method is expected to be rather efficient to describe A+BC→AB+C reactive collisions. In addition, the apparent complexity of the Hamiltonian expressed in these coordinates is reduced when applied to a wave packet represented in grids for the internal coordinates. The efficiency of this method as compared to the use of the most adapted Jacobi coordinates increases as the masses of the satellite atoms approach that of the heavier central atom and, what may be more important, avoids the problems associated with the singularities of the radial kinetic terms in the region of configuration space of interest. This method is used to study the Li+HF(v=0,1,j=0,J=0) reactive collision and the structure of the final state distribution of the LiF products is interpreted in terms of transition state resonances.

Potential energy surface and wave packet calculations on the Li+HF→LiF+H reaction

In this work an analytic fit of previous ab initio points [Aguado, Suarez, and Paniagua, Chem. Phys. 201, 107 (1995)] on the potential energy surface of the LiFH system is presented and the reaction dynamics is studied using a time-dependent treatment based on local coordinates. Three-dimensional wave packet calculations performed for zero total angular momentum indicate that the reactivity for the HF reactant in its ground vibrational state is quite low, in contradiction with previous dynamical calculations using different potential energy surfaces. The differences with previous potential energy surfaces are further analyzed using a bidimensional approach. Finally, the effect of the initial vibrational excitation of the HF reactant on the reactivity is studied using the bidimensional approach.

The lowest triplet state 3A′ of H3 +: Global potential energy surface and vibrational calculations

The adiabatic global potential energy surface of the H3+ system for the lowest triplet excited state of A′ symmetry was computed for an extensive grid of conformations around the minimum region at full configuration interaction ab initio level, using a much more extended basis set than in a previous paper from the same authors. An accurate global fit (rms error lower than 27 cm−1 for energies lower than dissociation into separated atoms and lower than 5 cm−1 for energies lower than the dissociation channel) to these ab initio points and also to part of the previous calculated points (for a total of 7689 energies in the data set) of the lowest triplet excited state of A′ symmetry is obtained using a diatomics-in-molecules approach corrected by one symmetrized three-body term with a total of 109 linear parameters and 1 nonlinear parameter. This produces an accurate global potential which represents all aspects of the bound triplet excited state of H3+ including the minima and dissociation limits, satisfying...