Condensed Phase Chemical Dynamics Research Articles

Interpolation for molecular dynamics simulations: from ions in gas phase to proteins in solution

The interpolation technique has shown many promises for simulating chemical dynamics with quantum chemical accuracy at molecular mechanics speed. This is achieved by constructing analytic potential energy surfaces with quantum chemical information at multiple conformational points, without assuming any functional form for the potentials. Here, we briefly review the course the method was developed over the past few decades, with a special focus on the activities in Korea. We also describe its strengths and weaknesses toward describing condensed phase chemical dynamics with the present implementations. Perspectives for future developments toward increasing applicability are discussed as concluding remarks. © 2015 Wiley Periodicals, Inc.

Semiquantal time-dependent Hartree approach to condensed phase chemical dynamics: application to the system-bath model.

A semiquantal analysis of condensed phase chemical dynamics, outlined recently for a double-well linearly coupled to dissipative harmonic bath, is formulated in detail to clarify its general features as well as the specifics of the linear and quadratic coupling cases. The theory may be called a "semiquantal time-dependent Hartree (SQTDH)" approach, as it assumes a factorized product of the squeezed coherent state wave packets for the variational subspace of the many-dimensional time-dependent wave function. Due to this assumption, it straightforwardly satisfies the canonicity condition introduced by Marumori et al. is described by a set of Hamilton equations of motion in an extended phase space that includes auxiliary coordinates representing the wave packet widths. The potential in the extended phase space provides a pictorial understanding of the quantum effects affected due to the bath coupling, e.g., suppression of the wave packet spreading in terms of the potential wall developing along the auxiliary coordinates. The idea is illustrated by prototypical models of quartic double-well and cubic metastable potentials linearly and quadratically coupled to the bath. Further applications and extensions, where the SQTDH method will offer a practical approach for introducing quantum effects into realistic molecular dynamics simulations, are also discussed.

Interrogation and control of condensed phase chemical dynamics with linearly chirped pulses: I2 in solid Kr

The effect of linearly chirped pulses in condensed phase ultrafast pump–probe experiments is investigated by classical simulations for the model system of I2 isolated in a Kr matrix. The central frequency of the probe laser is selected to monitor exclusively the event of first collision and recoil of atoms from the host cage. It is shown that a chirped probe pulse enables characterization of the magnitude and sign of the momentum of the evolving trajectory flux. This can be understood by transforming the frequency–time profile of the probe pulse to coordinate–time space, and noting that the observable signal is a function of the relative group velocities of the traveling wave packet and the traveling window function. The effect of the pump pulse chirp, is a measure of the controllability of the evolving dynamics. In the particular case studied, breaking and remaking of the I2 bond near the dissociation limit of the bare molecule, it is shown that the memory of the system outlasts the collision with the cage. Negatively chirped pulses produce a more tightly focused wave packet during recoil, leading to a stronger population coherence in the subsequent dynamics.

Energy Flow during Isomerization Reactions in Liquids

We introduce a method for trajectory analysis of energy flow during condensed phase chemical dynamics. The method requires specification of a reaction path and consists of monitoring the work done by the reaction path variable on various parts of the system. It is generally applicable to complex systems, is simple to calculate, and provides a consistent method of examining the role of a bath on energy relaxation dynamics. The method is illustrated by studying cyclohexane isomerization in liquid carbon disulfide.

Generalized Langevin theory for many-body problems in chemical dynamics: Modelling of solid and liquid state response functions

The equivalent harmonic chain heatbath modelling method [S. A. Adelman, J. Chem. Phys. 71, 4471 (1979)] for reducing problems in condensed phase chemical dynamics to effective few-body trajectory problems is tested via the calculation of typical solid and liquid state velocity response (correlation) functions χ̇ (t). Finite nearest neighbor harmonic chain models for condensed phase atomic dynamics which may be used in trajectory simulations, are constructed from moments of the spectral density of χ̇ (t) and from additional information about the small z behavior of e.g., diffusion coefficients. Model response functions are then generated numerically as specified trajectories of the chain equations of motion. The convergence of chain model response functions is investigated for the bulk elastic model (isotropic continuum elastic treatment) of a solid and for the Gaussian approximation to the Levesque–Verlet molecular dynamics results for a Lennard-Jones liquid. For both cases, the chain model heatbaths are found to give response functions which: (i) converge rapidly and systematically; (ii) exactly reproduce the chemically crucial short-time response and thus account for caging (in liquids) and initial chemical system/heatbath energy transfer; (iii) reproduce the chemically less important long time response in a qualitatively correct manner. It is thus concluded that the chain modelling techniques yield a chemically and calculationally useful reduction of condensed phase dynamics.