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A study on intermolecular proton transfer investigated quantum mechanical tunneling effects and rates using a new, dynamical, ab initio, semiclassical formalism. This new semiclassical method was applied to proton transfer in malonaldehyde. Proceeding entirely from first principles, the results obtained were in excellent agreement with the experimentally measured tunneling splitting. The new technique turned out to be very accurate in the test case used and is a very promising approach to examining proton transfer problems.

Focuses on an extension of the full multiple spawning method for quantum non-adiabatic dynamics that capitalizes on the global nature of quantum mechanics and the deterministic nature of the method. Use of a classically motivated time-dependent basis set for the wave function; Reduction of the scaling of the dominant effort in ab initio multiple spawning.

The problem of zero point energy in classical trajectory computations is discussed and illustrated by an example of dissociation where the zero point energy is used to provide the required energy. This is not possible in quantal dynamics. A proposed route to the alleviation of the problem, based on using classical-like trajectories which mimic the solution of the (expectation values) of Heisenberg equations of motion, is discussed. In general, one cannot simultaneously correct for all possible expectation values, so the remedy is at best partial. The variable whose expectation value and variance is to be handled correctly is examined in detail for a one-dimensional anharmonic potential, and is identified with the logarithmic derivative of the wave function in the Wentzel–Kramers–Brillouin (WKB) approximation. The multidimensional case is also discussed and it is pointed out that the zero point energy problem can be particularly severe for systems which exhibit a locally unstable classical motion. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The solvent-induced electronic predissociation [B→a1g(3Π)] following an ultrafast X→B transition in molecular iodine is studied using a classical ensemble representation of Heisenberg’s equations of motion. An N electronic state quantum mechanical Hamiltonian is used to derive (coupled) equations of motion for the population (and the coherence) of the different electronic states as well as classicallike coupled equations for the nuclear dynamics (of both the molecule and the solvent) on each electronic state. The ultrafast excitation of the intermediate B state creates a coherent vibrational motion in this bound state. The localized nature of the solvent-induced B–a1g(3Π) coupling results in a steplike depletion of the excited B state population and hence in a bulletlike appearance of population on the dissociative a1g(3Π) state twice per vibrational period. The depletion of the B state population and the appearance of products on the a1g(3Π) state are discussed as a function of solvent density and polarizability. The magnitude of the nonadiabatic B–a1g(3Π) coupling depends both on the molecule–quencher separation and on the quencher’s polarizability. It is found that at all reduced densities the small Ar atom is the most effective quencher (when compared to either Kr and/or Xe). We attribute this unexpected trend to the local density of atoms around the solute molecule. For all the rare gas solvents the local density around the iodine molecule does not quite scale with the global one and there is an observed tendency for the solvent to cluster around the solute in a T-shaped configuration. It is this close-packed configuration that compensates for the smaller polarizability of the Ar atom and hence provides for a more effective quenching. These arguments are used to explain the experimental results which demonstrate that for a series of homologous alkanes the extent of predissociation... [ABSTRACT FROM AUTHOR]

We discuss a classically-motivated method for modeling ultrashort laser pulse optical excitation. The very same method can be used to treat the breakdown of the Born–Oppenheimer approximation. The results are compared to numerically-exact quantum mechanics for a model problem representing excitation from the X (ground) state to the B (excited) state of molecular iodine. Expectation values and final B state populations are predicted quantitatively. The method provides a new way to simulate pump–probe experiments in particular and multistate dynamics in general. The method appears extendible to multidimensional problems. We argue that the increase of effort with dimensionality will be similar to that encountered in classical mechanical simulations as opposed to the exponential scaling of numerically-exact quantum mechanical propagation techniques. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A simple practical procedure which ensures that the energy in a molecular vibrational mode does not decrease below its zero-point value is discussed and applied. The method is based on taking the classical limit of the Hamiltonian and thereby deriving classical equations of motion which are solved via a standard classical trajectory computation. We refer to this as the ‘‘reference’’ trajectory. It is argued that the reference solution differs from what one would obtain if one were to begin with a classical description of the problem; the difference being that the reference computation puts the zero of energy at the correct, quantum-theoretic, zero, i.e., at the zero point. To obtain a fully classical-like solution one needs to shift the energy and period of the reference trajectory and the different ways of doing this are discussed. The resulting, energy, and phase shifted, equivalent classical trajectory cannot, by construction, lose the zero-point energy from the modes in which it is placed. The method is discussed first for the obvious case of a single oscillator, including the role of the anharmonicity, and is then applied to a variety of dimers [I2He, ArHBr, (HF)2] where a higher frequency mode is coupled to a low-frequency one and the problem is to prevent the (high) zero-point energy from being made available for transfer to the far weaker mode. Other advantages of the proposed scheme, such as the correct frequency dependence of the power spectrum, and its application to an unbound motion in the continuum are also discussed. [ABSTRACT FROM AUTHOR]

A three dimensional model Hamiltonian is used to mimic and interpret the results of full molecular dynamics simulations of an ion-molecule activationless recombination process in a solvent of structureless atoms. By making an adiabatic separation of variables it is shown that the gas phase capture model, suitably modified to incorporate the dynamical role of the solvent motion, can be used also in solution. Specifically, a motion along one uncoupled coordinate describes the capture process. The angular momentum for this coordinate is constant during the approach motion and thereby it provides a suitable criterion for capture. The motion of the approaching reactants is shown to be in the strong coupling adiabatic limit. In this limit there is a combination of two effects: A weak ion-molecule attractive interaction at large separations and a substantial solvation of the ion by the liquid. Thus the solvent is able to follow the motion along the reaction coordinate and to take part in the crossing of the centrifugal barrier. A second implication of the model is the efficient deactivation of the ion pair as a result of nonadiabatic V-T transitions. These transitions are confined to the ion-pair polarization well region, i.e., to the left of the adiabatic region of the centrifugal barrier. If a ‘‘solvent-separated’’ ion pair is formed the recombination process is delayed and the reorganization of the solvent is required to facilitate a successful capture. To model this effect a nonlinear, space dependent, coupling term is used in the model Hamiltonian. Comparison is made throughout between the results of full molecular dynamics simulations, computational results for the model Hamiltonian, and the predications of the adiabatic separation. The role of strong solvation in activationless recombination reactions is discussed in terms of the adiabatic separation and its breakdown. The conclusions are compared, and contrasted, with the case of... [ABSTRACT FROM AUTHOR]

A simple Hamiltonian model which accounts for the main features of the role of the solvent in activated bimolecular exchange reactions is discussed. The properties of the potential energy along the reaction coordinate of the solute enter in an essential way and explain the different roles of the solvent near the barrier and at the foothills of the potential and the corresponding separation of time scales. The activation energy necessary to surmount the barrier is provided by a localized, vibrationally nonadiabatic, energy exchange between the solvent and solute. Caging with and without recrossing of the barrier is discussed. The predictions of the model are compared with exact trajectory results for the given Hamiltonian and with full molecular dynamics simulations. The influence of the physical parameters such as masses, barrier height strength of solvent–solute coupling, etc., is well accounted for by the model and is summarized by two dimensionless coupling parameters. In particular, the efficiency of solvent solute energy exchange is governed by a vibrational adiabaticity parameter. [ABSTRACT FROM AUTHOR]

Focuses on the validation of full multiple spawning (FMS) molecular dynamics method for two dimensional electronic state Hamiltonian model. Theoretical background of FMS; Description of potential energy surfaces; Analysis of diabetic reaction probabilities.

Molecular dynamics simulations demonstrate facile dissociation of halogen molecules embedded in rare gas clusters upon impact at a surface at collision velocities up to 10 km/s. Two pathways are discerned: a heterogeneous dissociation of the molecule on the surface and a homogeneous mechanism where rare gas atoms which have rebounded from the surface cause the translational–vibrational coupling. The total yield of dissociation of the clustered molecule can reach up to 100%, whereas the yield of dissociation of the bare, vibrationally cold molecule saturates below 40%. A systematic study of the role of different conditions is made possible by not accounting for the atomic structure of the surface. The role of dissipation at the surface is found, however, to be quite important and is allowed for. Larger clusters, clusters of the heavier rare gases and a more rigid surface, all favor the homogeneous mechanism. Evidence for a shock front which, upon the initial impact, propagates into the cluster; the binary nature of the homogeneous dissociation process; and the absence of a dominant cage effect are discussed. A quantitative functional form of the velocity dependence of the yield of dissociation, which accounts for the size of the cluster, the rigidity of the surface and other attributes, is used to represent the data. The physics of the processes within the cluster is dominated by the novel dynamical features made possible when the duration of the atom–molecule collisions is short compared to the vibrational period. This ‘‘sudden’’ regime is sudden with respect to all modes of the nuclear motion and provides a hitherto unavailable tool for examination of reaction dynamics under extreme conditions. [ABSTRACT FROM AUTHOR]

Understanding the large-scale three-dimensional structure of the inner heliosphere, while important in its own right, is crucial for space weather applications, such as forecasting the time of arrival and propagation of coronal mass ejections (CMEs). This study uses sunRunner3D (3D), a 3-D magnetohydrodynamic (MHD) model, to simulate solar wind (SW) streams and generate background states. SR3D employs the boundary conditions generated by corona-heliosphere (CORHEL) and the PLUTO code to compute the plasma properties of the SW with the MHD approximation up to 1.1 AU in the inner heliosphere. We demonstrate that SR3D reproduces global features of corotating interaction regions (CIRs) observed by Earth-based spacecraft (OMNI) and the Solar Terrestial Relations Observatory (STEREO)-A for a set of Carrington rotations (CRs) that cover a period that lays in the late declining phase of solar cycle 24. Additionally, we demonstrate that the model solutions are valid in the corotating and inertial frames of references. Moreover, a comparison between SR3D simulations and in situ measurements shows reasonable agreement with the observations, and our results are comparable to those achieved by Predictive Science Inc.’s Magnetohydrodynamic Algorithm outside a Sphere (MAS) code. We have also undertaken a comparative analysis with the Space Weather Adaptive Simulation Framework for Solar Wind (SWASTi-SW), a PLUTO physics-based model, to evaluate the precision of various initial boundary conditions. Finally, we discuss the disparities in the solutions derived from inertial and rotating frames.

Research is presented concerning the observation of disrotatory ring-opening motion which has occurred as a result of excited state ab initio molecular dynamics simulations. The preference for Woodward-Hoffmann stereochemistry is discussed.

The ab initio multiple spawning method was used to calculate the absorption and resonance Raman excitation profiles of ethylene after varPi-varPi excitation.

Full multiple spawning method is used in investigating the photoinduced cis-trans isomerization of all-trans retinal protonated Schiff base (RPSB). The study seeks to find out if equilibrium is a condition for directed excitation transport given an electronic funnel. RPSB is known for fast quenching of less than 500 fs which guarantees that equilibrium among the excited electronic states cannot be achieved before electronic quenching. Results show quenching and exciton transfer are two distinct events although the latter is a highly directed event.