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  • 1
    Electronic Resource
    Electronic Resource
    Quantum mechanical integral cross sections and rate constants for the F+HD reactions (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 9802-9809 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this article we report on the first accurate quantum mechanical temperature-dependent rate constants for the two possible products of the (F+HD) system and on the corresponding intramolecular kinetic isotope effects. The calculations were done for the Stark–Werner and the Hartke–Stark–Werner potential energy surfaces. It was found that the two surfaces yield significantly different rate constants for both products but similar molecular kinetic isotope effects. These isotope effects are about two times larger than the experimental ones, at the lowest measured temperature region (160–200 K) but become rather close to them at ∼400 K. The F+HD is known to exhibit, at the low energy region, various kinds of isotope effects. In the present study we revealed a new isotope effect related to the dependence of (integral) cross sections on the initial rotational states ji at intermediate energies. Whereas the cross sections to form DF are only mildly dependent on ji (at most 20% for j0=4) a very large effect—which at some energies (∼0.1 eV) enlarges the integral cross sections almost three times—is obtained for HF. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481618
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  • 2
    Electronic Resource
    Electronic Resource
    The semirigid vibrating rotor target model for atom-polyatom reaction: Application to H+H2O→H2+OH (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 585-591 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The semirigid vibrating rotor target (SVRT) model for the polyatomic reaction has been applied to the reaction of H+H2O→H2+OH using the time-dependent wave packet approach. Since the SVRT model for a general atom–polyatom reaction involves only four-mathematical dimensions (4D), the SVRT dynamics calculation for H+H2O requires much less computational effort than the exact full-dimensional treatment. Numerical calculation shows that by properly choosing the values for the excluded degrees of freedom, excellent results are obtained for the computed reaction probability, cross section, and rate constant. The present numerical calculation for H+H2O reaction from the initial ground state clearly demonstrates that the SVRT model for the polyatomic reaction provides an accurate and practical approach for computational study of chemical reactions involving polyatomic molecules. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480551
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  • 3
    Electronic Resource
    Electronic Resource
    Effects of reagent rotation on the dynamics of the H2+OH reaction: A full dimension quantum study (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 2708-2716 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have extended the time-dependent wave packet method to calculate cross sections and rate constants for rotationally excited initial states by using the centrifugal sudden (CS) approximation. A detailed study of the effects of rotational excitation of reagents on the title reaction on the WDSE PES has been carried out. It is found that (a) OH rotational excitation very mildly enhances the total cross section, (b) H2 rotational excitation quite substantially reduce the cross section, and (c) simultaneous OH and H2 rotational excitation has a largely uncorrelated effect. As a result, we found that the thermal rate constant can be obtained fairly accurately by only taking into account the effect of H2 rotation. A model calculation by changing the mass of an O atom reveals that the weak dependence of the cross section on OH rotation is not because the O atom is left relatively stationary by OH rotation. We speculate that it may be a general feature for the diatom-diatom reaction that the nonreactive diatom acts as a spectator not only vibrationally but also rotationally. It was also found that the "J-shifting" approximation works quite well for the reaction. On the other hand, the effect of K on the dynamics is found to be much stronger and more complicated than the J effect, making the "K-shifting" approximation not good for the reaction. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476881
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  • 4
    Electronic Resource
    Electronic Resource
    The cumulative reaction probability for the H2 + OH reaction (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 551-563 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The cumulative reaction probability [CRP or N(E)] for the four-atom reaction, H2+OH→H+H2O is calculated using one of the formulations of Miller, Schwartz, and Tromp [J. Chem. Phys. 79, 4889 (1983)] and the transition state wave packet (TSWP) approach of Zhang and Light [J. Chem. Phys. 104, 6184 (1996)]. It is shown that locating the dividing surface of the flux operator in the transition state region significantly reduces the number of wave packets which must be followed in order to converge the CRP as compared to the use of initial state selected wave packets (ISSWP). In addition we examine the use of transition state normal coordinates (versus Jacobi coordinates) and show that the use of transition state wave packets defined in normal coordinates yields more rapid convergence of the CRP and individual contributions of the TSWP to the CRP can closely approximate the probabilities of reaction for each transition state as a function of energy. Problems with large amplitude motions using the normal coordinates of the loose non-linear transition state are shown to be absent if normal coordinates of a linear transition state are used. Applications to the 3-D H + H2 (J = 0) reaction and to the 6D H2 + OH (J = 0) reaction demonstrate that both N(E) and the initial state reaction probabilities at many energies can be evaluated accurately and efficiently by propagation of each TSWP only once. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473394
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  • 5
    Electronic Resource
    Electronic Resource
    Transition state wave packet study of hydrogen diffusion on Cu(100) surface (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 5741-5753 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The transition state wave packet (TSWP) approach to the thermal rate constant based on the flux-flux autocorrelation function is used to investigate the diffusion dynamics of an H atom on the Cu(100) surface in the uncorrelated hopping regime. The high efficiency of the approach makes it feasible to include up to eight Cu modes explicitly in the time dependent quantum simulation. This is necessary since on the rigid surface the flux-flux autocorrelation function never decays to a negligibly small value to give a converged rate constant. For short times, the Cu modes included dynamically merely have a zero-point-energy effect on the flux-flux autocorrelation function. For longer times, however, the Cu modes absorb the activation energy of the H atom and effectively suppress recrossing of the transition state surface, resulting in convergence of the autocorrelation function and the hopping rate. For this system, recrossing of the transition state surface is minimal with the medium damping present, and the converged hopping rate can be well approximated by the short time behavior of the correlation function on the rigid surface. In addition, we find that the contributions of the excited Cu modes to the hopping rate may be accurately modeled by thermal "transition state" factors. Based on this, a new quantum transition state theory (QTST) is derived. The new theory provides a general way to calculate the approximate quantum correction to the traditional TST. It also provides a systematic and flexible tool to calculate the rate constant at any desired level of accuracy between the traditional TST level and the exact result. Finally, since the surface relaxation due to the presence of the H atom lowers both the energies of H atom in the binding well and on the saddle point almost equally, it only minimally affects the hopping rate, provided the configuration of the surface atoms is fully relaxed initially. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479870
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  • 6
    Electronic Resource
    Electronic Resource
    Fully converged integral cross sections of diatom-diatom reactions and the accuracy of the centrifugal sudden approximation in the H2+OH reaction (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 4435-4444 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The initial state selected time-dependent wave packet method has been extended to calculate integral cross sections for diatom-diatom chemical reactions without the CS (centrifugal sudden) approximation by including all important K (the projection of the total angular momentum on the body-fixed axis) blocks in the body-fixed frame. We report the first fully converged cross section for the ground rovibrational state of the title reaction and present a detail study of the accuracy of the CS approximation to the reaction. We find that for the ground rovibrational state the CS approximation works very well, but its accuracy deteriorates with increasing reagent rotational excitation. As expected, and as found in atom-diatom reactions, the CS approximation works much better in high energy region than in low energy region. In low energy region, the coupled channel cross sections are larger than the CS ones for all the rotationally excited states investigated here, in particular for the highly excited states. It is found the CS approximation gives rise to about 10% error in H2 or OH rotationally averaged rate constant. If simultaneous OH and H2 rotational excitation does not have a correlated effect on dynamics, the CS approximation introduces about 19% error in thermal rate constant for the reaction for low temperatures which is considerably larger than what is expected of a few percent. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478327
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  • 7
    Electronic Resource
    Electronic Resource
    Quantum rate constants for the H2+OH reaction with the centrifugal sudden approximation (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 79-86 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The cumulative reaction probability (CRP) has been calculated for the H2+OH↔H2O+H in its full dimensionality by using the centrifugal sudden (CS) approximation for J〉0. The Boltzmann average of the CRP provides the most accurate thermal rate constant to date for the title reaction on the Walch, Dunning, Schatz, Elgersma (WDSE) potential energy surface (PES). It is found that the theoretical rate is larger than the experimental value in the low temperature region (a factor of ∼1.8 at 300 K), and smaller than the experimental value for temperatures higher than 500 K, indicating that a more accurate PES is needed to provide a quantitative description of the title reaction. We also demonstrate that the "J-shifting" approximation in which we calculate N(J〉K,K) from N(J=K,K) by an energy shift works very well for this reaction. However, the "J- and K-shifting" approximation [calculating N(J,K) from N(J=0,K=0)] overestimates the rate for this reaction by about 60% for all the temperatures investigated. It is also found that the CS rate constant is substantially lower than the rate constant for the ground rovibrational state of the reagents calculated on the same PES, indicating that initial rotational excitation is important to the thermal rate constant for this reaction (it causes a decrease). © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476542
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  • 8
    Electronic Resource
    Electronic Resource
    Effects of reagent rotation and the accuracy of the centrifugal sudden approximation in the H2+CN reaction (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 203-211 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper presents fully converged integral cross sections for the ground rovibrational state and some rotationally excited initial states for the title reaction on the TSH3 PES. The initial state selected time-dependent wave packet method has been employed in the calculation with all important K blocks in the body-fixed (BF) frame included. We find that CN rotational excitation up to j2=7 essentially has no effect on the integral cross section, while H2 rotational excitation substantially reduces the cross section. As a result, the thermal rate constant can be obtained accurately by only taking into account the effect of H2 rotational excitation. It is found that the resulting thermal rate constant is considerably smaller than the initial state selected rate constant for the ground rovibrational state. It is also smaller than the experimental rate constant by a factor of 3 and 30% at T=209 K and 447 K, respectively, indicating the TSH3 PES used in the calculation is not quantitatively accurate in describing the reaction. In addition, we examine in detail the accuracy of the centrifugal sudden (CS) approximation to the reaction. Comparison between this reaction and the H2+OH reaction is also carried out when possible. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480572
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  • 9
    Electronic Resource
    Electronic Resource
    Cumulative reaction probability via transition state wave packets (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 6184-6191 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new time-dependent approach to the cumulative reaction probability, N(E), has been developed based on the famous formulation given by Miller and co-workers [J. Chem. Phys. 79, 4889 (1983)], N(E)=[(2π)2/2] tr[δ(E−H)Fδ(E−H)F]. Taking advantage of the fact that the flux operator has only two nonzero eigenvalues, we evaluate the trace efficiently in a direct product basis of the first flux operator eigenstates and the Hamiltonian eigenstates on the dividing surface (internal states). Because the microcanonical density operator, δ(E−H), will eliminate contributions to N(E) from an internal state with the energy much higher than the total energy E, we can minimize the number of internal states required by choosing a dividing surface with the lowest density of internal states. If the dividing surface is located in an asymptotic region, one just needs to include all the open channels, i.e., with internal energy lower than the total energy. Utilizing the Fourier transform for δ(E−H), we can obtain the information for all the energies desired by propagating these wave packets once. Thus the present approach will be much more efficient than the initial state selected wave packet (ISSWP) approach to N(E) for systems with many rotation degrees of freedom because the density of states in asymptotic region for such systems is much higher than that in the transition state region. With the present method one can also calculate the cumulative reaction probability from an initial state (or to a final state) by locating the second flux operator in the corresponding asymptotic region. This provides an alternative to the ISSWP approach which may be more efficient if the reaction probabilities from a large number of initial states are desired. The method is applied to the 3D H + H2 (even rotation) reaction for J=0 by locating the first dividing surface in the transition state region. The demonstration also shows an aspect less than ideal; the contribution to N(E) from a wave packet may be slightly larger than 1 or slightly smaller than 0, making it improper to interpret the contribution as a probability. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471302
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  • 10
    Electronic Resource
    Electronic Resource
    Potential inversion via variational generalized inverse (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 9713-9720 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The determination of potential energy surfaces (PES) from values calculated ab initio at a set of points or from spectral data (vibration–rotation energy level information and rotation constants) are important and often difficult problems. The former is a "potential interpolation'' problem, the latter a "potential inversion'' problem. These are indeterminate problems in which the known data is insufficient to yield a unique solution. We present here a new constrained variational approach to the direct solution of these problems. The constraints are the known exact values of the potential or the exact perturbation corrections desired. The variational functional is chosen to provide control of the magnitude and smoothness of the correction function or potential. The method is very simple, very fast computationally, and yields exact solutions to the perturbation or interpolation equations in a single application. The potential inversion is completed by iteration to converge the perturbation solutions for a given set of assigned levels, and then by repeating with additional levels assigned in sequence to the data set to yield a physically acceptable PES very quickly. This procedure yields a smooth PES from which the energy levels in the dataset are calculated exactly. Information on rotational constants may also be used. Both interpolation and inversion procedures are applied to the the two dimensional (R,θ) PES for ArOH(A 2Σ+). A combined application of these two procedures is also presented in the paper, where we first interpolate a PES from ab initio points and then correct the ab initio fitted surfaces using spectral data. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469934
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