Alfredo Aguado

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 ...

Reference program for molecular calculations with Slater-type orbitals

A program for computing all the integrals appearing in molecular calculation with Slater‐type orbitals is reported. The program is mainly intended as a reference for testing and comparing other algorithms and techniques. An analysis of the performance of the program is presented, paying special attention to the computational cost and the accuracy of the results. Results are also compared with others obtained with Gaussian basis sets of similar quality. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1284–1293, 1998

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.

Differential Cross Sections and Product Rotational Polarization in A + BC Reactions Using Wave Packet Methods: H+ + D2 and Li + HF Examples

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 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.

New program for molecular calculations with Slater‐type orbitals

A new program for computing all the integrals appearing in molecular calculations with Slater-type orbitals (STO) is reported. This program follows the same philosophy as the reference pogram previously reported but introduces two main changes: Local symmetry is profited to compute all the two-electron integrals from a minimal set of seed integrals, and a new algorithm recently developed is used for computing the seed integrals. The new code reduces between one and two orders of magnitude the computational cost in most polyatomic systems.

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.