The decoherence of an anharmonic oscillator in a thermal harmonic bath is examined via a semiclassical approach. A computational strategy is presented and exploited to calculate the time dependence of the purity and the decay of individual matrix elements in the energy representation for a variety of initial states. The time dependence of the decoherence is found to depend on the temperature of the bath, the coupling strength, the initial state of the oscillator, and the choice of quantity measuring the decoherence. Recurrences in the purity and in the off-diagonal matrix elements are observed, as well as the collapse of these matrix elements to the diagonal, providing evidence for the retention of quantum coherence for time scales longer than that indicated by the purity. The results are used to analyze the utility of the Caldeira-Leggett and Redfield models of decoherence and to assess the dependence of dephasing rates on the degree of structure in phase space. In several cases we find that the dephasing dynamics can be described as an initial Zeno-effect regime, followed by a Caldeira-Leggett region, followed by recurrences.

A method is presented that greatly improves the efficiency of semiclassical initial value representation treatments by transforming phase space integration variables to time, energy, and additional coordinates and momenta on a Poincare surface. Since the integration over time can be treated as an integration along the classical motion, the number of trajectories needed to obtain convergence is significantly reduced. The technique is applied to test cases involving bounded motion with very encouraging results.

A new, uniform, semiclassical, initial value representation expression is obtained for the S-matrix in the case of collinear collisions. The derivation is based on an asymptotic analysis ͑for large inter-fragment distances͒ of a uniform semiclassical integral expression for the time independent scattering wave function. Although this derivation specifically treats the case of the collision of an atom with a harmonic diatom, the final expression is generalized to arbitrary collinear collisions. The various properties of the expression and its relation to existing semiclassical methods are discussed. Numerical tests are performed for the well-known Secrest-Johnson system. Among other important advantages, the present treatment is a well-defined, uniform, semiclassical approximation that is capable of good accuracy and high computational efficiency, requiring a relatively small number of classical trajectories to obtain converged S-matrix elements for a given energy and initial state.

Vibrational decoherence of a "breathing sphere" oscillator in a thermal Lennard-Jones bath is examined using a classical analog approach. The equivalence between this approach and the linearized semiclassical initial value representation (IVR) is established and the method is exploited to produce a useful computational strategy that can efficiently evaluate the time dependence of the decoherence in these systems. A comparison between Harmonic and Morse "breathing sphere" models is presented and the rate of decoherence is found to depend on the choice of model, the initial state of the oscillator, the initial conditions of the bath (temperature, density), and the choice of quantity measuring the decoherence rate. The results are used to examine the utility of the Caldeira-Leggett model in this realistic system.

We generalize a recently-developed semiclassical uniform initial value representation (IVR) treatment of the S-matrix [Y. Elran and K. G. Kay, J. Chem. Phys. 114, 4362 (2001)] to chaotic nonreactive and reactive collinear scattering. The present modifications allow one to determine the phase of the complex IVR integrand in a unique and practical manner even when the integrand is discontinuous or rapidly varying. The method is applied to the collinear H+H2 exchange reaction on the Porter–Karplus surface. A strategy is introduced for adapting the integration over the chaotic chattering zones to the fractal nature of the integrand. The results indicate that the technique is capable of good accuracy while requiring a relatively small number of trajectories per energy.