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 error was reduced to 0.07 kcal/mol and the maximum absolute deviation to 0.25 kcal/mol (approximately one half the corresponding values of the LSTH ones). The new global fitting procedure is then applied to obtain potential energy functions for the H2He+ and the LiHF systems within the chemical accuracy of 1 kcal/mol.

Adiabatic global potential energy surfaces, for singlet and triplet states of AЈ and AЉ symmetries, were computed for an extensive grid for a total of 8469 conformations of H 3 ϩ 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 Ϫ1) 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 H 3 ϩ 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 Jϭ20 using hyperspherical coordinates with symmetry adapted basis functions. The infrared spectra thus reproduced is within 1 cm Ϫ1 with respect to the experimental values for several transitions.

Exact quantum stereodynamics: The steric effect for the Li+HF→LiF+H reactionIn 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 ͑1997͔͒ for which more ab initio points have been calculated ͑from 644 to 2323͒. 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 Jϭ5 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.

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.

The effect of the reagent initial state excitation on the reactive cross section in the LiϩHF(v, j) collision is analyzed for vϭ0, 1 and jϭ0, 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 ͑1997͔͒. The reaction cross-section for vϭ0 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 vϭ1 the reaction cross-section increases by a factor of 10-50 with respect to that of vϭ0. The influence of the alignment of the initial angular momentum on the reaction cross section is studied.