Rob van Harrevelt

Accurate quantum calculations of the reaction rates for H∕D+CH4

In previous work [T. Wu, H. J. Werner, and U. Manthe, Science 306, 2227 (2004)], accurate quantum reaction rate calculations of the rate constant for the H+CH4-->CH3+H2 reaction have been presented. Both the electronic structure calculations and the nuclear dynamics calculations are converged with respect to the basis sets employed. In this paper, the authors apply the same methodology to an isotopic variant of this reaction: D+CH4-->CH3+HD. Accurate rate constants are presented for temperatures between 250 and 400 K. For temperatures between 400 and 800 K, they use a harmonic extrapolation to obtain approximate rate constants for H/D+CH4. The calculations suggest that the experimentally reported rate constants for D+CH4 are about a factor of 10-20 too high. For H+CH4, more accurate experiments are available and agreement is much better: the difference is less than a factor of 2.6. The kinetic isotope effect for the H/D+CH4 reactions is studied and compared with experiment and transition state theory (TST) calculations. Harmonic TST was found to provide a good description of the kinetic isotope effect.

The reaction rate for dissociative adsorption of N2 on stepped Ru(0001): six-dimensional quantum calculations.

Quantum-mechanical calculations of the reaction rate for dissociative adsorption of N2 on stepped Ru(0001) are presented. Converged six-dimensional quantum calculations for this heavy-atom reaction have been performed using the multiconfiguration time-dependent Hartree method. A potential-energy surface for the transition-state region is constructed from density-functional theory calculations using Shepard interpolation. The quantum results are in very good agreement with the results of the harmonic transition-state theory. In contrast to the findings of previous model calculations on similar systems, the tunneling effect is found to be small.

Multidimensional time-dependent discrete variable representations in multiconfiguration Hartree calculations

In the multiconfiguration time-dependent Hartree (MCTDH) approach, the wave function is expanded in time-dependent basis functions, called single-particle functions, to increase the efficiency of the wave-packet propagation. The correlation discrete variable representation (CDVR) approach, which is based on a time-dependent discrete variable representation (DVR), can be employed to evaluate matrix elements of the potential energy. The efficiency of the MCTDH method can be further enhanced by using multidimensional single-particle functions. However, up to now the CDVR approach could not be used in MCTDH calculations employing multidimensional single-particle functions, since this would require a general multidimensional non-direct-product DVR scheme. Recently, Dawes and Carrington presented a practical scheme to implement general non-direct-product multidimensional DVRs [R. Dawes and T. Carrington, Jr., J. Chem. Phys. 121, 726 (2004)]. The present work utilizes their scheme in the MCTDH/CDVR approach. The accuracy is tested using the photodissociation of NOCl as example. The results show that the CDVR scheme based on multidimensional time-dependent DVRs allows for an accurate evaluation of the potential in MCTDH calculations with multidimensional single-particle functions.

Quantitative spectroscopic and theoretical study of the optical absorption spectra of H2O, HOD, and D2O in the 125–145 nm region

The room temperature absorption spectra of water and its isotopomers D2O and HOD have been determined in absolute cross section units in the 125 to 145 nm wavelength region using synchrotron radiation. The experimental results for these B band spectra are compared with results from quantum mechanical calculations using accurate diabatic ab initio potentials. A Monte Carlo sampling over the initial rotational states of the molecules is applied in order to calculate the cross sections at a temperature of 300 K. The overall rotation of the water molecule is treated exactly. Both for the experimental and for the theoretical spectrum an analysis is made in terms of a component attributed to rapid direct dissociation processes and a component attributed to longer-lived resonances. The agreement between the results from experiment and theory is excellent for H2O and D2O. In the case of HOD in the results of theory two more resonances are found at low energy. It is demonstrated that the width of the resonances of 0.04 eV is the result of overlapping and somewhat narrower resonances in the spectra of molecules differing in rotational ground state.

Quantum mechanical calculations for the H2O + hnu --> O(1D) + H2 photodissociation process.

Quantum mechanical wavepacket calculations for the photodissociation of water in the second absorption band are presented. Using O + H2 Jacobi coordinates, partial cross sections for the O(1D) + H2 channel are calculated for different initial rotational states. Conical intersection and Renner-Teller effects are included. The branching ratios for the four accessible dissociation channels at 121.6 nm are in good agreement with experiment (J. Chem. Phys. 1982, 77, 2432). The calculations predict significant rotational and vibrational excitation of the H2 fragments. Photodissociation of ortho and para water produces predominantly, but not exclusively, ortho and para H2 fragments, respectively.

Degeneracy in discrete variable representations: General considerations and application to the multiconfigurational time-dependent Hartree approach

Problems appear in discrete variable representations (DVRs) based on general basis sets when the coordinate matrix has degenerate eigenvalues. Then the DVR is not uniquely defined. This paper shows that this problem can be caused by symmetry. Taking the symmetry into account when constructing the DVR solves the problem. The symmetry effect can be particularly important for the time-dependent DVR used in multiconfigurational time-dependent Hartree calculations employing the correlation DVR (CDVR) approach. Problems reported previously for the initial-state selected treatment of the H+H(2) reaction can be attributed to this symmetry effect. They can be solved by using a symmetry-adapted approach to construct the time-dependent DVR. Thus, the present paper shows that the CDVR scheme can be employed also in initial-state selected scattering calculations if the symmetry of the system is properly taken into account in the construction of the time-dependent DVR.

Off-normal incidence dissociative sticking of H2 on Cu(100) studied using six-dimensional quantum calculations

Six-dimensional quantum calculations of the sticking probability for H2 hitting a Cu(100) surface with off-normal incidence are presented. The multiconfiguration time-dependent Hartree approach is employed for an efficient wave-packet propagation. The sticking probability is calculated for different initial momenta parallel to the surface. In contrast with the picture described in the literature, the sticking probability was found to depend on the parallel momentum. The results are explained by the topology of the potential-energy surface, which shows significant corrugation with a moderate variation of the barrier height with the surface site.

The effect of surface relaxation on the N2 dissociation rate on stepped Ru: A transition state theory study

The interaction between N2 and the surface results in a considerable rearrangement of the structure of the surface near the steps. The present work investigates the effect of this interaction on the rate of dissociative adsorption within the framework of harmonic transition state theory. Employing different models where selected Ru degrees of freedom are included, we show that the dominant effect of surface relaxation is a shift of the classical barrier height. Therefore, the “relaxed/N2” model, which takes relaxation of the surface into account but neglects vibrations of the surface atoms explicitly, can be used for reliable rate constant calculations. Sticking coefficients calculated employing the relaxed/N2 model based on different density-functional theory (DFT) calculations are then compared with experiment. Comparison with the experiment suggests that for the best DFT calculations the barrier height (0.49eV) is about 0.15eV too high.