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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 2598-2604 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Potential energy surface points computed from variants of density functional theory (DFT) are used to calculate directly the anharmonic vibrational frequencies of H2O, Cl−H2O, and (H2O)2. The method is an adaptation to DFT of a recent algorithm for direct calculations of anharmonic vibrational frequencies using ab initio electronic structure codes. The DFT calculations are performed using the BLYP and the B3LYP functionals and the results are compared with experiment, and also with those calculated directly from a potential energy surface obtained using ab initio Möller-Plesset second–order perturbation theory (MP2). The direct calculation of the vibrational states from the potential energy points is performed using the correlation-corrected vibrational self-consistent field (CC-VSCF) method. This method includes anharmonicity and correlations between different vibrational modes. The accuracy of this method is examined and it is shown that for the experimentally measured transitions the errors in the CC-VSCF calculations are much less than the errors due to the potential energy surface. By comparison with the experimentally measured frequencies the CC-VSCF method thus provides a test for the quality of the potential energy surfaces. The results obtained with the B3LYP functional, in contrast to those of the BLYP functional, are of comparable quality to those obtained with MP2. The B3LYP anharmonic frequencies are in good agreement with experiment, showing this DFT method describes well the anharmonic part of the potential energy surface. The BLYP results systematically underestimate both the harmonic and anharmonic frequencies and indicate that using this functional for the description of hydrogen-bonded systems may cause significant errors. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 8763-8768 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method for the treatment of correlation effects between modes in vibrational self-consistent-field (VSCF) calculations is introduced. It is based upon using a partially separable form for the wave function. As a result, some of the modes are treated as mutually fully correlated, while the rest are separable. The modes which are explicitly coupled together in the calculation are chosen on physical grounds. Trial calculations are performed upon H2O, H3O+, and CH3NH2 and indicate that the method performs well. The agreement with experiment for the explicitly coupled modes is improved when compared to both the vibrational self-consistent-field method and its correlation-corrected extension. When interfaced with an electronic structure code this method opens the way for the accurate first-principles prediction of vibrational frequencies of strongly coupled modes. If only a few modes are mutually strongly coupled, the method has a very favorable scaling with system size, as does VSCF itself. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 6046-6056 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method for approximate solution of the time-dependent vibrational Schrödinger equation, applicable to extended molecular systems, is presented. The new method is essentially an approximate time-dependent quantization of classical dynamics. A molecular dynamics simulation is used to obtain a separable, effective time-dependent potential for each mode, that implicitly includes also the effects of all the other modes on this degree of freedom. A time-dependent wave packet is then propagated separately for each mode, using the corresponding effective potential. The new approximation is valid for short time scale processes only, but it is easily applicable to large realistic systems. Test calculations against exact quantum and time-dependent self-consistent field (TDSCF) results are carried out for two examples; photodissociation of HI in the collinear Xe...HI cluster, and electron photodetachment from the collinear Ar...I−...Ar cluster. For illustration, the new scheme is also applied to photodetachment from large linear clusters Arn...I−...Arn (n=2–8) and the results are discussed. For the test systems, the results of the new method are virtually identical to those following from the computationally much more demanding TDSCF approach, and they are in excellent agreement with the exact results. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 8855-8864 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A recently developed method for time-dependent quantum simulations of large systems on short time scales is applied to the dynamics following electron photodetachment from the clusters I−(Ar)2 and I−(Ar)12. The problem is treated in full dimensionality, incorporating all vibrational degrees of freedom, by the classically based separable potential (CSP) approach. This is essentially an approximate time-dependent quantization of classical dynamics: Classical molecular dynamics is used to generate effective, single mode separable time-dependent potentials for each degree of freedom. The quantum dynamics is then propagated separately for each mode, using the effective potentials that implicitly include effects such as energy transfer between the modes. In the current application of the CSP method we calculate properties relevant for the interpretation of spectroscopies, such as correlation functions of wave packets, as well as time-dependent atom–atom distribution functions, pertinent to future diffraction experiments using ultrafast pulses. The insight obtained from the quantum dynamics of these clusters is discussed. In particular, light is thrown on the differences in the dynamics associated with the system landing on the three different electronic surfaces of the neutral I(2P)⋅(Ar)n system. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 1975-1987 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general method for studying transition state spectroscopy and dynamics in hydrogen atom transfer reactions is presented. This approach is based on the time-dependent self-consistent field (TDSCF) approximation and is applied to a study of the ClHCl− photodetachment experiments of Metz et al. [Metz et al., J. Chem. Phys. 88, 1463 (1988)]. Comparison of results of exact time-dependent and TDSCF calculations are made for collinear and three-dimensional (J=0) approximations for the quantum dynamics. When ClHCl is constrained to be collinear, the TDSCF calculation overcorrelates the motions in the H atom displacement and ClCl extension coordinates. This results in relatively poor agreement with the exact result for many properties of the wave function. In contrast, when the system is propagated in the three vibrational coordinates of the system, the transition state dynamics are effectively over much more rapidly. Consequently, the TDSCF approximation yields results of very good quantitative accuracy over the time required for most of the wave function to decay off of the transition state. Comparison is also made between the wave function that results from the exact propagation and from TDSCF when the wave function in the ClCl stretch coordinate is approximated by a Gaussian wave packet. Here the magnitude of the overlap between the two TDSCF wave functions in the H atom coordinates, for quantum and semiclassical propagations of the wave function in the ClCl distance coordinate, is greater than 0.98 over the time of the propagations. These TDSCF calculations are repeated for a wave function that is approximated by a product of a two-dimensional wave function in the hydrogen atom coordinates and a one-dimensional wave function in the ClCl extension coordinate and even better quantitative agreement with the exact propagation is achieved. The success of this method for studying ClHCl gives us confidence that TDSCF will provide a general powerful tool for studies of hydrogen and proton transfer reactions in large systems.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 343-355 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The role of solvent effects in association reactions is studied in atom-cluster collisions. Classical trajectory studies of the systems H+Cl(Ar)n (n=1,12) are used to investigate the influence of size, structure, and internal energy of the "microsolvation'' on the H+Cl association reaction. The following effects of solvating the chlorine in an Arn cluster are found. (1) In the H+ClAr system there is a large "third body'' effect. The single solvent atom stabilizes the newly formed HCl molecule by removing some of its excess energy. The cross section found at low energies is a substantial fraction of the gas-kinetic cross section. The molecule is produced in highly excited vibrational-rotational states. (2) Some production of long-lived HCl...Ar complexes, with lifetimes of 1 ps and larger, is found for the H+ClAr collisions. Weak coupling stemming from the geometry of the cluster is the cause for long life times. These resonance states decay into HCl+Ar. (3) At low collision energy (E=10 kJ/mol) for H+Cl(Ar)12, the H+Cl association shows a sharp threshold effect with cluster temperature. For temperatures T≥45 K the cluster is liquidlike, and the reaction probability is high. For T≤40 K the cluster is solidlike, and there is no reactivity. This suggests the potential use of reactions as a signature for the meltinglike transition in clusters. (4) At high collision energies (E=100 kJ/mol) H atoms can penetrate also the solidlike Cl(Ar)12 cluster. At this energy, the solid–liquid phase change is found not to increase the reaction probability.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 4056-4062 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociation dynamics of the cluster Li(H2)2, following the 2s→2p excitation of the Li atom, is studied in the framework of a collinear model. The process was investigated by exact quantum wave packet calculations, and the results were used to test a hybrid quantum/classical method, in which the highly quantum mechanical initial state of the cluster is described by a wave function, and the latter is used to sample initial positions and momenta for a classical treatment of the excited state dynamics. We found that the dynamics was dominated by two predissociation processes, Li*(H2)2→Li*–H2+H2 and Li*(H2)2→Li*+(H2)2, with the latter process having a higher yield. A relatively long dissociation lifetime, ∼10 ps, was found for the excited cluster. The slow vibrational predissociation rate was interpreted as due to the very low density of state involved. The hybrid quantum/classical approach was found to give product vibrational energy and velocity distributions in good accord with the distribution from exact calculation. However, the lifetimes from the hybrid approach were found to be much shorter than those from the exact quantum calculations. The hybrid approach is thus applicable even to photoexcitation of quantum clusters for studying certain selected properties. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2577-2591 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure and stability of clusters of a boron atom with one to eight H2 molecules is investigated. For the simplest BH2 clusters, the lowest ab initio adiabatic potentials for o-H2 and p-H2 interacting with a boron atom are used. For the larger clusters (n=2–8), the p-H2 is treated as a sphere, and the total potential is taken to be the sum of pairwise additive B–H2 and H2–H2 interactions which include, in the former case, an anisotropy due to the orientation of the unpaired B 2p electron. This electronic interaction is considerably more attractive when H2 approaches the B atom in a plane perpendicular to the orientation of the 2p orbital. The local and global minima on these potential surfaces were located and diffusion quantum Monte Carlo simulations were used to determine the energies and properties of the ground state wave functions for these B–(H2)n clusters. For the B(H2) cluster, a comparison is made with the results of variational calculations.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1823-1829 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An algorithm for first-principles calculation of vibrational spectroscopy of polyatomic molecules is proposed, which combines electronic ab initio codes with the vibrational self-consistent field (VSCF) method, and with a perturbation-theoretic extension of VSCF. The integrated method directly uses points on the potential energy surface, computed from the electronic ab initio code, in the VSCF part. No fitting of an analytic potential function is involved. A key element in the approach is the approximation that only interactions between pairs of normal modes are important, while interactions of triples or more can be neglected. This assumption was found to hold well in applications. The new algorithm was applied to the fundamental vibrational excitations of H2O, Cl−(H2O), and (H2O)2, using the Möller–Plesset method for the electronic structure. The vibrational frequencies found are in very good accord with experiments. Estimates suggest that this electronic ab initio/VSCF approach should be feasible, with reasonable computational resources, for all-mode calculations of vibrational energies and wave functions for systems of up to 10–15 atoms. The new method can be also very useful for testing the accuracy of electronic structure codes by comparing with experimental vibrational spectroscopy. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 4355-4366 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method for calculating decay rates of vibrational modes in large polyatomic systems is proposed and tested. The high frequency excited vibration is treated quantum mechanically, and the soft modes are described classically. The initial state is described by the hybrid quantum/classical self-consistent-field (SCF) approximation. The formalism is based on a golden-rule expression. The driving potential is the difference between the full Hamiltonian and the mean field Hamiltonian (SCF) causing the decay of the initial state to final mixed quantum/classical SCF states. These states are calculated using an extension of the usual static mean-field techniques to systems with mixed quantum and classical degrees of freedom. The formalism for obtaining the mean-field states and calculating the decay rates is presented, and the method is applied to a diatomic molecule treated quantum mechanically, embedded in a 1D model for a rare gas cluster treated classically. The dependence of the eigenenergies of the quantum and the decay rates with temperature is studied. The influence on the system size is also presented and compared with the prediction of the isolated binary collision model. The effect of a change in the linear density of the cluster on the eigenenergies of the vibrational mode is presented.
    Type of Medium: Electronic Resource
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