Time-dependent Quantum Wave Packet Calculation Research Articles

Quantum Dynamics Study of Torsional Excitation of Glycine in Collision with Hydrogen Atom on ab Initio Potential Energy Surface

Quantum mechanical study has been carried out to investigate C−C torsional excitation in glycine via collisional energy transfer with hydrogen atom. In this study, ab initio calculation is carried out to generate potential energies on a two-dimensional grid (torsional angle and radial coordinate) for given fixed orientational angle (θ, φ). These discrete energies are fitted with a local linear least-squares method to generate potential energies at any given point in two-dimensional space for dynamics calculation. Time-dependent quantum wave packet calculation is employed to study energy transfer to the C−C torsional mode of glycine, and state-to-state transition probabilities are obtained for different initial angles of collision. Strong angle-dependent energy transfer is observed from the calculation. Although the total energy transferred to the torsional mode is small, collision from certain angles can result in relatively large conformational change in torsion by as much as 30° degrees.

Accurate quantum mechanical calculation for the N+OH reaction

Accurate three-dimensional time-dependent quantum wave packet calculations for the N+OH reaction on the A″3 potential energy surface [Guadagnini, Schatz, and Walch, J. Chem. Phys. 102, 774 (1995)] have been carried out. The calculations show for the first time that the initial state-selected reaction probabilities are dominated by resonance structures, and the lifetime of the resonance is generally in the subpicosecond time scale. The calculated reaction cross sections indicate that they are a decreasing function of the translational energy, which is in agreement qualitatively with the quasiclassical trajectory calculations. The rate constants obtained from the quantum mechanical calculations are consistent with the quasiclassical trajectory results and the experimental measurements.

Quantum reactive scattering calculations of cross sections and rate constants for the N(2D)+O2(X 3Σg−)→O(3P)+NO(X 2Π) reaction

Time-dependent quantum wave packet calculations have been performed on the two lowest adiabatic potential energy surfaces (2 2A′ and 1 2A″) for the N(2D)+O2(X 3Σg−)→O(3P)+NO(X 2Π) reaction. The calculations have been carried out, on these recently published potential energy surfaces, using the real wave packet method together with a new dispersion fitted finite difference technique for evaluating the action of the radial kinetic energy operator. Reaction probabilities, corresponding to the O2 reactant in its ground vibrational-rotational state, have been calculated for both surfaces and for many different values of the total angular momentum quantum number (J), within the helicity decoupling approximation. The reaction probabilities associated with all other relevant J values have been interpolated, and to a smaller extent extrapolated, using a capture model, to yield probabilities as a function of energy. The probabilities have in turn been summed to yield energy dependent cross sections and then used to compute rate constants. These rate constants are compared with ones obtained from quasiclassical trajectory (QCT) and variational transition state theory (VTST) calculations performed on the same surfaces. There is a good agreement between the wave packet and QCT cross sections for reaction on both potential energy surfaces considered, with the exception of the near threshold region, where the reaction probability is dominated by tunnelling. Comparison of the predicted rate constants shows that for the 2 2A′ surface, above 300 K, the wave packet, QCT and VTST results are quite similar. For the 1 2A″ surface, however, significant differences occur between the wave packet and the other methods. These differences become smaller with increasing temperature. It is likely that these differences arise, at least in part, from the fact that, when calculating the rate constants, the reactants are restricted to be in their lowest vibrational-rotational state in the wave packet calculations but are selected from a thermally equilibrated population in the other methods.

Calculations of the F + HD reaction on three potential energy surfaces

In this paper, the three-dimensional time-dependent quantum wave packet calculation has been employed to study the non-adiabatic reaction dynamics of F + HD on three, electronically non-adiabatic potential energy surfaces fitted by Alexander, Stark and Werner (J. Chem. Phys., 2000, 113, 11 084; named ASW). The integral cross-sections and branching ratio of the reaction as a function of collision energy are calculated. The integral cross-sections are compared with the experimental measurements and other theoretical results. For the reaction of F in its excited state with HD, the calculated function of integral cross sections as collision energy shows that a steplike feature near 0.9 kcal mol−1 probably arises from the collinear FHD geometry.

Time-dependent quantum wave packet study of H+HCN→H2+CN reaction

Time-dependent quantum wavepacket calculations for the H+HCN reaction are carried out on the ab initio potential energy surface of ter Horst et al. [J. Chem. Phys. 105, 558 (1996)]. The dynamics calculations are performed using both the semirigid vibrating rotor target (SVRT) model [J. Chem. Phys. 111, 3929 (1999)] as well as the pseudo atom–diatom model. Total reaction probabilities from the initial ground state of the reagent are calculated for various values of the total angular momentum quantum number J. Reaction cross sections and rate constants are also calculated. The dynamical result from the SVRT calculation is compared with that from a pseudo atom–diatom calculation in which the HCN is treated as a pseudo diatom. Both the SVRT and pseudo atom–diatom calculations involve three degrees of freedom for the H+HCN reaction due to linearity of the HCN molecule at both reactant and transition states. The results from these two calculations are generally close to each other with some difference at high collision energies. The two models for the current system are essentially the same except that the rotational constant used is different. In particular, the SVRT model uses the correct rotational constant for the linear HCN molecule while the pseudo atom–diatom model produces a rotational constant which is much larger than the correct one.

Time-Dependent Quantum Dynamics Study of the C + CH Reaction on the 2A‘ Surface

Three-dimensional time-dependent quantum wave packet calculation was carried out to study the dynamics of the C + CH reaction on the 2A ' potential energy surface. The energy dependence of the calculated total reaction probabilities exhibits no long-lived resonance despite the presence of a deep well, a feature similar to that on the 1A(2) surface. The effect of initial vibrational and rotational excitation of the reagent on reactivity is investigated, and the reaction rate constants and integral cross sections are calculated and compared with quasiclassical trajectory results. This study shows that results of the quantum calculation are generally in good agreement with the QCT results and the difference is relatively small. The analysis of stereodynamics indicates that the C + CH reaction could occur within a wide range of the attack angle. The C side of the product CH (molecule or radical) is the slight favorite side for the reactive attack.

Theoretical investigation of [formula omitted] vibronic-coupling and ultrafast internal-conversion dynamics in the acetylene cation

The dynamics and spectroscopy of the X 2Π u and A ̃ 2Σ g + states of C 2H 2 + and C 2D 2 + have been investigated in the framework of a vibronic-coupling model. The model includes six of the seven vibrational degrees of freedom and has been constructed on the basis of CASSCF/CASPT2 ab initio electronic-structure calculations. The main feature of the model is a conical intersection of the 2Σ g + state with the 2Π u state. Time-dependent quantum wave-packet calculations have been performed for this model and photoelectron spectra have been obtained by Fourier transformation of the so-called autocorrelation functions. The calculations provide the explanation of the observed extremely short lifetime of the A ̃ 2Σ g + state in terms of an ultrafast internal-conversion process caused by the 2Σ g +– 2Π u conical intersection. The calculated photoelectron spectra and autocorrelation functions are compared with experimental data of Reutt et al. [J. Chem. Phys. 84 (1986) 3022]. The comparison provides evidence that the ultrafast dynamics of the 2Σ g + state is affected by an additional curve crossing involving the 2Π g shake-up state.

Time-dependent quantum wave packet study of the C+CH reaction

The quantum dynamics of the C+CH reaction has been studied by means of time-dependent quantum wave packet calculation on 1A′ potential energy surface. Initial state-specific total reaction probabilities and integral cross section are calculated, and the rate constants obtained are in reasonably good agreement with experimental measurement. Although a deep well is present in the potential surface, no long-lived resonances are found from the dynamics calculation and the reaction proceeds by a direct mechanism, similar to that of O(1D)+H2 reaction. Stereodynamics study of the reaction is carried out by transforming the scattering result to stereodynamics (SD) representation at zero total angular momentum. The SD result shows that the C+CH reaction proceeds from almost all attack angles with a preference for attacking at the C side of CH (molecule or radical) by the carbon atom.

Quantum dynamics calculation of reaction probability for H + Cl2 → HCl + Cl

We present in this paper a time-dependent quantum wave packet calculation of the initial state selected reaction probability for H + Cl2 based on the GHNS potential energy surface with total angular momentumJ = 0. The effects of the translational, vibrational and rotational excitation of Cl2 on the reaction probability have been investigated. In a broad region of the translational energy, the rotational excitation enhances the reaction probability while the vibrational excitation depresses the reaction probability. The theoretical results agree well with the fact that it is an early down-hill reaction.

Quantum dynamics study of the Cl+D2 reaction: Time-dependent wave packet calculations

The quantum dynamics of the Cl+D2 reaction has been studied by means of time-dependent quantum wave packet calculations on the G3 and BW2 potential energy surfaces. Initial state-specific total reaction probabilities and integral cross sections are calculated, and the thermal rate constant is obtained. On the G3 surface, the effect of the reagent’s rotational excitation on the reactivity is negative, while on the BW2 surface, the rotation of reagent has positive effect on the reactivity. Comparison of the thermal rate constants on the G3 and BW2 surfaces with experimental measurement is shown.

Time-Dependent Quantum Dynamics Study of the Cl + HD Reaction

The dynamics of the Cl + HD reaction has been studied by means of time-dependent quantum wave packet calculations on the G3 and BW2 potential energy surfaces. Initial state-specific total reaction probabilities and integral cross sections are calculated, and the thermal rate constant is obtained. The good agreement was shown in the comparison of our integral cross sections with other quantum mechanics calculations on the same PES. The thermal rate constants on the G3 surface are close to the experiment findings. In the case of neglect of spin−orbit coupling in the ab initio calculations of the BW2 surface, the thermal rate constants on the surface are somewhat higher than the experimental measurement. When the spin−orbit coupling is taken into account, the thermal rate constants on the BW2 surface are close to the experiment as well.

Time-dependent quantum dynamics study of reactive scattering of the HD+CN system in the potential averaged 5D model

Time-dependent (TD) quantum wavepacket calculation for the HD+CN reaction has been carried out on a new potential energy surface (denoted TSH3) with the potential averaged five dimensions (PA5D) model. Discrete variable representations (DVR) for radial coordinate and a renormalized angular quadrature scheme are applied in the wavepacket propagation in order to save computer memory. We employ an energy projection method to extract reaction probabilities for a whole range of energies from the single wavepacket propagation. The influence of the initial rovibrational effects to the reaction probabilities is discussed. The reaction probabilities together with the cross-section and rate constant results are given.

Time-dependent quantum studies of the O(3P)+HCl(X 1Σ+) reaction

Time-dependent quantum wave packet calculations are carried out for the O(3P)+HCl reaction using the Koizumi, Schatz, and Gordon (KSG) and the Ramachandran, Senekowitsch, and Wyatt (RSW) potentials for the total angular momentum J=0. A detailed study is performed on the KSG surface for which reaction probabilities for the formation of the OH product as a function of collision energies are reported for the v0=0,1 and j0⩽10 states of HCl. The present results are in good agreement with previous time-independent quantum treatments, and physical insights into the reaction dynamics based on an analysis of the calculated wave packet are explored, for example, mechanisms for the rotationally enhanced reaction probabilities that have been observed at both v0=0 and 1. The most profound effects are observed when HCl is initially in states with j0>5 or v0=1. A comparison of the results for v0=0, j0=0 with those obtained from the RSW surface indicates that the width of barrier and the topology of the potential play crucial roles in governing the dynamics of the O(3P)+HCl reaction.

Time-Dependent Quantum Dynamics Study of Reactive Scattering of the H + O2 Involving Long-Lived Resonances

A time-dependent quantum wave packet calculation for the combustion reaction H+O2 using double many-body expansion IV potential energy surface was studied. The resonance-like structures of probabilities together with the cross section and rate constant results are given.

A comparison of quantum and classical dynamics of H 2 scattering from Cu(111)

In this work we report results for the scattering of H 2 molecules from a system showing dissociative adsorption. We compare the results of time-dependent quantum wavepacket calculations with those obtained using classical mechanics. It is found that the trends in the scattering dynamics are broadly accounted for by the classical simulations. A notable exception to this is found for initial molecules having non-zero angular momentum.

Dissociative adsorption of H2: Time-dependent quantum studies

In this short review we consider some selected results for the dissociation of hydrogen on metal surfaces. In particular we focus our attention on the effects that surface corrugation have on the dissociation and also how rotations and vibrations couple in the interaction region. Results from time dependent quantum wavepacket calculations have been particularly helpful in forming our ideas of how diatomics dissociate. We describe this method along with two examples of propagation schemes. The power of limited dimensional models to rationalize experimental observations is clearly demonstrated.

Quantum dynamics study of the reaction HD+OH→H+DOH, D+HOH

Accurate time-dependent (TD) quantum wavepacket calculations are reported for the combustion reaction HD+OH. Due to the lack of symmetry, the HD+OH reaction has roughly twice the number of channels of the corresponding H2+OH reaction and produces two distinguishable products–HOH and HOD. In order to make the TD calculation possible on workstations with limited memories, we employed a normalized quadrature scheme in the wavepacket propagation by the split-operator propagator. The normalized quadrature scheme eliminates the need to store large matrices during the wavepacket propagation while preserving the unitarity of the split-operator propagator and producing numerically stable results. This approach made TD dynamics calculations possible on small-memory workstations for the title reaction and for other polyatomic reactions. Reaction probabilities, cross sections, rate constants, and reaction branching ratios are reported in this paper for the title reaction. The observed strong dependence of the reaction probabilities on the reactive HD rotation and the relative weak dependence on the nonreactive OH rotation are explained in terms of a steric effect. The isotope effect in the branching ratio is examined and physical explanation is given for the observed branching ratio at low and high kinetic energies.

Quantum reactive scattering with a deep well: Time-dependent calculation for H+O2 reaction and bound state characterization for HO2

We show in this paper a time-dependent (TD) quantum wave packet calculation for the combustion reaction H+O2 using the DMBE IV (double many-body expansion) potential energy surface which has a deep well and supports long-lived resonances. The reaction probabilities from the initial states of H+O2(3Σ−g) (v=0–3, j=1) for total angular momentum J=0 are obtained for scattering energies from threshold up to 2.5 eV, which show numerous resonance features. Our results show that, by carrying out the wave packet propagation to several picoseconds, one can resolve essentially all the resonance features for this reaction. The present TD results are in good agreement with other time-independent calculations. A particular advantage of the time-dependent approach to this reaction is that resonance structures—strong energy dependence of the reaction probability—can be mapped out in a single wave packet propagation without having to repeat scattering calculations for hundreds of energies. We also report calculations of some low-lying vibrational energies of the hydroperoxyl radical HO2(2A″) and their spectroscopic assignments. The vibrational frequencies of HO2(2A″) on the DMBE IV potential energy surface are lower than experimental values, indicating the need to further improve the accuracy of the potential energy surface.

Product state distribution in time-dependent quantum wave packet calculation with an optical potential

We present a simple and efficient approach for extracting the final state-specific information in time-dependent quantum wave packet calculations for scattering or half-scattering problems without the artificial boundary reflection. The method, which is based on the compactness of the interaction picture wavefunction, employs an optical potential to absorb the flux of the Schrödinger picture wavefunction near the edge of the boundary. However, the absorption does not disturb the interaction picture wavefunction which is used to compute the stable product state distributions. Two numerical examples using this approach are given. One is for a model collinear photodissociation of CH 3I involving nonadiabatic coupling, and the other is for a rigid rotor scattered off a flat surface.