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  • 1
    Electronic Resource
    Electronic Resource
    A remedy for zero-point energy problems in classical trajectories: A combined semiclassical/classical molecular dynamics algorithm (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 2034-2038 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method is proposed for dealing with difficulties in molecular dynamics (MD) simulations caused by nonpreservation of zero-point energies (ZPE) in classical dynamics. Specifically addressed is a difficulty, for molecules held in weakly bound clusters, of energy flow from the initial ZPE of stiff molecular vibrations into soft cluster modes, causing unphysical dissociation or melting of the cluster. The remedy proposed is a classicallike MD algorithm, which treats the stiff modes by semiclassical Gaussian wave packets and the soft modes by classical dynamics, using the time-dependent self-consistent field (TDSCF) approach to couple the classical and the semiclassical modes. The resulting algorithm is very similar in form to classical MD, is computationally simple, stable, and appears free of unphysical effects. The method is illustrated by test applications to models of the clusters I2He and (HBr)2 in the ground states, which dissociate at the expense of their ZPE classically, but remain stable in the new method.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462106
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  • 2
    Electronic Resource
    Electronic Resource
    Mixed quantum wave packet/classical trajectory treatment of the photodissociation process ArHCl→Ar+H+Cl (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 7242-7250 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation of HCl in the cluster Ar...HCl by an extremely short pulse was studied using a hybrid quantum mechanical/classical approach. In this method, the H atom is treated quantum mechanically, the heavy atoms classically, and the time-dependent self-consistent-field (TDSCF) approximation is used to couple the quantum with the classical modes. The results are compared with those of classical trajectory calculations. On the whole, good qualitative agreement is found between the results of the (partly quantum) hybrid method and the pure classical ones. However, quantum interference effects of quantitative significance are found both in the angular and in the kinetic energy distribution of the H atom product. These effects, and resonances that contribute to the process, are analyzed in terms of wave packets obtained for the H atom in the hybrid method. The usefulness and applicability of the hybrid method are discussed in the light of the results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463550
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  • 3
    Electronic Resource
    Electronic Resource
    Dissociation dynamics of I2⋅⋅⋅Nen van der Waals clusters (n=1–9): A quasiclassical approach (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7868-7874 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics of vibrational predissociation (VP) of van der Waals (vdW) I2⋅⋅⋅Nen clusters is studied by means of a quasiclassical trajectory approach. We assume a near-equilibrium geometry that accounts for the I2 stretch and the stretching and bending modes of the rare gas atoms restricted to move along a plane perpendicular to the I2 axis. A sharp increase of the halfwidth is observed for n=9 together with certain stability of the n=8 cluster size with respect to dissociation. It would indicate the existence of a first coordination shell for Ne around I2 containing eight atoms. In addition, the dissociation results less and less efficient as n increases and statistical mechanisms become important for the cluster sizes studied.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460122
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  • 4
    Electronic Resource
    Electronic Resource
    Structure and dynamics of small I2 . . . Hen van der Waals clusters (n=1–9) (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 6504-6513 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Energetics and dynamics of van der Waals (vdW) I2⋅⋅⋅Hen clusters are studied in an approximate way by using a model that considers 2n+1 degrees of freedom, that is, the I2 stretch and the 2n stretching and bending modes of the He atoms restricted to move along a plane perpendicular to the I2 axis. For n=2,3 a configuration-interaction treatment is carried out to obtain energy levels. For n=4–8, ground-level energies are estimated from those corresponding to n=2,3 and the geometric relationships among the n vdW bonds. A quasiclassical trajectory approach is used to study the dynamics of these clusters, and lifetimes and half-widths for vibrational predissociation have been calculated for n=1–9. A large increase in the half-width of n=9 with respect to the cases n=1–8 is observed, which would imply the existence of a first coordination shell for He about I2 containing eight atoms. Also, it is found that the mechanisms of dissociation for these clusters become statistical as the number of vdW bonds increases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458285
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  • 5
    Electronic Resource
    Electronic Resource
    Hybrid quantum/semiclassical wave packet method for molecular dynamics: Application to photolysis of Ar...HCl (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 427-436 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A hybrid quantum/semiclassical method is proposed and applied to study realistically the dynamics of the three-fragment photodissociation process Ar...HCl+hν→Ar+H+Cl. In the method the hydrogen motion is treated by exact quantum mechanics, while the heavy atoms are described by semiclassical Gaussian wave packets. This treatment is expected to reproduce the main quantum features of the dynamics. Part of the wave packet is found to describe resonance events in which the light particle is temporarily trapped inside the Ar...Cl cage and oscillates periodically between the heavy atoms before it dissociates. Interference between frequency components of the H wave function that populate different resonance levels give rise to interesting quantum effects. Such effects appear in the angular distribution of the hydrogen fragment, which shows some diffraction oscillations, and scattering into classically forbidden regions. Quantum interferences between the resonances are also the cause of a pronounced structure of peaks in the H photofragment kinetic energy distribution (KED). Time-correlation functions of the wave functions involved are computed, and the implications for the absorption spectrum and its relation to the KED of the H atom are discussed. The results demonstrate the power and applicability of quantum/semiclassical time-dependent self-consistent-field (TDSCF) as a tool for studying the dynamics and spectroscopy of realistic molecular systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464636
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  • 6
    Electronic Resource
    Electronic Resource
    An approximate quantal treatment to obtain the energy levels of tetra-atomic X ⋅⋅⋅ I2 ⋅⋅⋅ Y van der Waals clusters (X,Y=He,Ne) (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 496-507 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure of tetra-atomic X ⋅⋅⋅ I2 ⋅⋅⋅ Y van der Waals (vdW) clusters, where X,Y=He,Ne, is studied using an approximate quantal treatment. In this model the above complexes are treated as like diatomic molecules with the rare-gas atoms playing the role of electrons in conventional diatomics. Then a H2-like molecular-orbital formalism is applied, choosing the discrete states of triatomic systems I2 ⋅⋅⋅ X(Y) as molecular orbitals. Calculations at fixed configurations as well as including vdW bending motions restricted to the plane perpendicular to the I2 axis have been carried out for the sake of comparison with previous results. Finally, the restrictions are relaxed and the vdW bending motions are incorporated in a full way within the framework of a configuration interaction. The structure of these clusters is also studied through the probability density function.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458451
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  • 7
    Electronic Resource
    Electronic Resource
    Effects of solvation by a single atom on photodissociation: Classical and quantum/classical studies of HCl photolysis in Ar⋅⋅⋅HCl (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 3297-3306 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photolysis of HCl in the cluster Ar⋅⋅⋅HCl is studied theoretically with the objective of elaborating on the effect of a single "solvent'' atom on the dynamics of chemical bond breaking. The focus is on observable properties, such as the velocity distribution and the angular distribution of the H atom product, on how these properties reflect the "solvent'' effect, and on the physical mechanisms involved. The main results obtained are the following. (1) There is a high probability for at least a single "hard'' collision between the H photofragment and the Ar atom before the H atom leaves the cluster. Multiple collisions between the H and the heavy atoms also occur with significant probability. (2) The final kinetic-energy distribution of the H atom shows a long pronounced tail due to energy transfer in the collisions with the heavy atoms. (3) There are pronounced peaks in the angular distribution of the H atom due to the single and multiple collision events. (4) Comparison of photolysis of Ar⋅⋅⋅HCl with that of Ar⋅⋅⋅DCl shows a large isotope effect, again due to collisions within the cluster during its fragmentation. The results were mostly obtained from classical trajectory calculations, but also in part from calculations using a hybrid quantum/classical method in which the H atom is treated by quantum wave packets while the heavy atoms are described classically. The quantitative results show some quantum effects, but for most purposes the classical description is sustained. Implications of the results for experimental studies of molecular photodissociation in clusters are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463018
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  • 8
    Electronic Resource
    Electronic Resource
    Quantum-mechanical study of the I2–Ne vibrational predissociation dynamics: A three-dimensional time-dependent self-consistent-field approach (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 1047-1055 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A three-dimensional time-dependent self-consistent-field (TDSCF) approach is proposed to study the vibrational predissociation of the I2(B)–Ne van der Waals (vdW) complex. Jacobian coordinates are used within the assumption of zero-total angular momentum. In the method the total wave function is factorized such that the bending mode of the system is explicitly separated, while the coupling between the remaining two degrees of freedom is treated exactly. The decay dynamics of several resonances corresponding to different initial vibrational states of I2 is investigated through long-time wave packet propagations. Calculated resonance lifetimes are compared to experimental data and found to be in quantitative agreement with them. The results show that predissociation of the complex is mainly governed by the coupling between the I2 and the vdW stretching vibrations, whereas the bending mode has a rather weak effect on the dynamics. The good quality of the TDSCF description of this long-time dynamical process is due to the adaptation of the decoupling approximations applied in the method to the physical situation of the system. The validity of the approach is discussed in the light of the results. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470830
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  • 9
    Electronic Resource
    Electronic Resource
    Three-dimensional quantum wave packet study of the Ar–HCl photodissociation: A comparison between time-dependent self-consistent-field and exact treatments (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 3463-3473 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The time-dependent self-consistent-field (TDSCF) approximation is used to study the photodissociation of the Ar–HCl cluster in a three-dimensional framework. The results are compared with numerically exact quantum calculations, and the properties and accuracy of the TDSCF approach are evaluated on this basis. The TDSCF approximation is used in Jacobi coordinates, and the total wave function is factorized into a wave packet for two coordinates associated with the H atom, and a wave packet for a single coordinate that describes the relative motion of the heavy particles. Quantitative agreement between the TDSCF and the exact results is found for most quantities calculated. The calculations show that photodissociation, and in particular the departure of the H atom is predominantly a direct process, but an appreciable amount of wave packet amplitude moving in excited state resonances is also found. This amplitude seems significantly larger than obtained in recent calculations by Schröder et al. [J. Chem. Phys. 100, 7239 (1994); Chem. Phys. Lett. 235, 316 (1995)]. The validity and computational efficiency of the TDSCF approach for realistic systems of this type is discussed. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470230
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  • 10
    Electronic Resource
    Electronic Resource
    Photolysis of HCl in the Ar2-HCl and Ar-HCl Clusters: The Cluster Size Effect (1994)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 98 (1994), S. 3518-3526 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100064a039
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