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Uwe Manthe

Publications and source records attributed to Uwe Manthe.

13 recordsLinked to original sources
PubMed2006

Photoionization-induced dynamics of ammonia: ab initio potential energy surfaces and time-dependent wave packet calculations for the ammonia cation.

An analytical anharmonic six-dimensional three-sheeted potential energy surface for the ground and first excited states of the ammonia cation has been developed which is tailored to model the ultrafast photoinduced dynamics. Selected ab initio cuts, obtained by multireference configuration interaction calculations, have been used to determine the parameters of a diabatic representation for this Jahn-Teller and pseudo-Jahn-Teller system. The model includes higher-order coupling terms both for the Jahn-Teller and for the pseudo-Jahn-Teller matrix elements. The relaxation to the ground state is possible via dynamical pseudo-Jahn-Teller couplings involving the asymmetric bending and stretching coordinates. The photoelectron spectrum of NH3 and the internal conversion dynamics of NH3+ have been determined by wave packet propagation calculations employing the multiconfigurational time-dependent Hartree method. Three different time scales are found in the dynamics calculations for the second absorption band. The ultrafast Jahn-Teller dynamics of the two excited states occurs on a 5 fs time scale. The major part of the internal conversion to the ground state takes place within a short time scale of 20 fs. This fast internal conversion is, however, incomplete and the remaining excited state population does not decay completely even within 100 fs.

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PubMed2006

Accurate potential energy surface and quantum reaction rate calculations for the H+CH4-->H2+CH3 reaction.

Calculations for the cumulative reaction probability N(E) (for J=0) and the thermal rate constant k(T) of the H+CH(4)-->H(2)+CH(3) reaction are presented. Accurate electronic structure calculations and a converged Shepard-interpolation approach are used to construct a potential energy surface which is specifically designed to allow the precise calculation of k(T) and N(E). Accurate quantum dynamics calculations employing flux correlation functions and multiconfigurational time-dependent Hartree wave packet propagation compute N(E) and k(T) based on this potential energy surface. The present work describes in detail the various convergence test performed to investigate the accuracy of the calculations at each step. These tests demonstrate the predictive power of the present calculations. In addition, approximate approaches for reaction rate calculations are discussed. A quite accurate approximation can be obtained from a potential energy surface which includes only interpolation points on the minimum energy path.

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PubMed2005

Calculating initial-state-selected reaction probabilities from thermal flux eigenstates: a transition-state-based approach.

An approach for the calculation of initial-state-selected reaction probabilities utilizing a transition-state view and the multiconfigurational time-dependent Hartree approach is presented. Using flux correlation functions, wave packets located in the transition-state region are constructed and propagated into the asymptotic region to obtain initial-state-selected reaction probabilities. A complete set of reaction probabilities is obtained from a single set of thermal flux eigenstates. Concepts previously applied with success to the calculation of k(T) or N(E) are transferred to the calculation of state-selected probabilities. The benchmark H+H(2) (J=0) reaction on the LSTH potential-energy surface is used to test the reliability of this approach.

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PubMed2005

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.

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PubMed2005

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.

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PubMed2005

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.

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PubMed2004

First-principles theory for the H + CH4 --> H2 + CH3 reaction.

A full-dimensional quantum dynamics simulation of a hydrogen atom reacting with methane on an accurate ab initio potential energy surface is reported. Based on first-principles theory, thermal rate constants are predicted with an accuracy comparable to (or even exceeding) experimental precision. The theoretical prediction is within the range of the significantly varied experimental rate constants reported by different groups. This level of accuracy has previously been achieved only for smaller, three-or four-atom reactive systems. Comparison with classical transition state theory confirms the importance of quantum mechanical tunneling for the rate constant below 400 kelvin.

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PubMed2004

The ground state tunneling splitting of malonaldehyde: accurate full dimensional quantum dynamics calculations.

Benchmark calculations of the tunneling splitting in malonaldehyde using the full dimensional potential proposed by Yagi et al. are reported. Two exact quantum dynamics methods are used: the multiconfigurational time-dependent Hartree (MCTDH) approach and the diffusion Monte Carlo based projection operator imaginary time spectral evolution (POITSE) method. A ground state tunneling splitting of 25.7+/-0.3 cm(-1) is calculated using POITSE. The MCTDH computation yields 25 cm(-1) converged to about 10% accuracy. These rigorous results are used to evaluate the accuracy of approximate dynamical approaches, e.g., the instanton theory.

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PubMed2004

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.

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PubMed2004

Multiconfigurational time-dependent Hartree calculations for dissociative adsorption of H2 on Cu(100).

The efficiency of the multiconfigurational time-dependent Hartree (MCTDH) method for calculating the initial-state selected dissociation probability of H(2)(v=0,j=0) on Cu(100) is investigated. The MCTDH method is shown to be significantly more efficient than standard wave packet methods. A large number of single-particle functions is required to converge the initial-state selected reaction probability for dissociative adsorption. Employing multidimensional coordinates in the MCTDH ansatz (mode combination) is found to be crucial for the efficiency of these MCTDH calculations. Perspectives towards the application of the MCTDH approach to study dissociative adsorption of polyatomic molecules on surfaces are discussed.

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PubMed2004

Photoinduced dynamics of ethene in the N, V, and Z valence states: a six-dimensional nonadiabatic quantum dynamics investigation.

The photoinduced dynamics of ethene following pi-->pi(*) excitation is investigated by quantum wave-packet dynamics on three coupled six-dimensional diabatic potential-energy surfaces representing the N, V, and Z valence states, which have been developed previously. The C-C stretching and torsion, as well as the pyramidalization and scissoring of both CH(2) groups are included in this description. The wave-packet calculations have been performed using the multiconfigurational time-dependent Hartree method for a time period up to 100 fs. While a small amount of population transfer to the electronic ground state is found within this period, the overall population decay time of the V state is found to exceed the 100 fs range significantly. The autocorrelation function of the wave packet and the stationary absorption spectrum of the V state also have been calculated. It is found that both the torsional mode as well as the C-C stretching mode contribute to the very extended vibrational structure of the absorption spectrum, and that both modes are strongly coupled. At least on the present ab initio surface of limited dimensionality, the speed of pyramidalization of 90 degrees twisted ethene appears as the bottleneck for the ultrafast radiationless decay of the V state.

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