Time dependent semiclassical tunneling through one dimensional barriers using only real valued trajectories.

The time independent semiclassical treatment of barrier tunneling has been understood for a very long time. Several semiclassical approaches to time dependent tunneling through barriers have also been presented. These typically involve trajectories for which the position variable is a complex function of time.

Surface hopping, transition state theory and decoherence. I. Scattering theory and time-reversibility.

We provide an in-depth investigation of transmission coefficients as computed using the augmented-fewest switches surface hopping algorithm in the low energy regime. Empirically, microscopic reversibility is shown to hold approximately. Furthermore, we show that, in some circumstances, including decoherence on top of surface hopping calculations can help recover (as opposed to destroy) oscillations in the transmission coefficient as a function of energy; these oscillations can be studied analytically with semiclassical scattering theory.

Improving the efficiency of Monte Carlo surface hopping calculations.

A surface hopping method with a Monte Carlo procedure for deciding whether to hop at each step along the classical trajectories used in the semiclassical calculation is discussed. It is shown for a simple one-dimensional model problem that the numerical efficiency of the method can be improved by averaging over several copies of the sections of each trajectory that span the interaction regions. The use of Sobol sequences in the selection of the initial momentum for the trajectories is also explored.

The calculation of multidimensional semiclassical wave functions in the forbidden region using real valued coordinates.

A method that uses only real valued coordinates is presented for integrating the many dimensional semiclassical wave function into the forbidden region. The procedure first determines a surface of caustic points by running the set of trajectories that define the wave function in the allowed region. In the forbidden region, the momentum and the action integral are both complex functions of position, and their imaginary parts vanish on the caustic surface.

A singularity free surface hopping expansion for the multistate wave function.

A version of a surface hopping wave function for nonadiabatic multistate problems, which is free of turning point singularities, is derived and tested. The primitive semiclassical form of the particular surface hopping method considered has been shown to be highly accurate, even for classically forbidden processes. However, this semiclassical wave function displays the usual singular behavior at turning points and caustics in the classical motion.

Semiclassical nonadiabatic surface-hopping wave function expansion at low energies: hops in the forbidden region.

The accuracy of a semiclassical surface-hopping expansion of the time-independent wave function for problems in which the nonadiabatic coupling is peaked in the classically forbidden regions is studied numerically for a one-dimensional curve-crossing problem. This surface-hopping expansion has recently been shown to satisfy the Schrodinger equation to all orders in h and all orders in the nonadiabatic coupling. It has also been found to provide very accurate transition probabilities for problems in which the crossing points of the diabatic energy surfaces are classically allowed.

A semiclassical model for the calculation of nonadiabatic transition probabilities for classically forbidden transitions.

A semiclassical surface hopping model is presented for the calculation of nonadiabatic transition probabilities for the case in which the avoided crossing point is in the classically forbidden regions. The exact potentials and coupling are replaced with simple functional forms that are fitted to the values, evaluated at the turning point in the classical motion, of the Born-Oppenheimer potentials, the nonadiabatic coupling, and their first few derivatives. For the one-dimensional model considered, reasonably accurate results for transition probabilities are obtained down to around 10(-10).

An analysis through order variant Planck's constant over 2pi(2) of a surface hopping expansion of the nonadiabatic wave function.

It is shown that a surface hopping expansion of the semiclassical wave function formally satisfies the time independent Schrodinger equation for many-state, multidimensional problems. This wave function includes terms involving hops between different adiabatic quantum states as well as momentum changes without change of state at each point along classical trajectories. The single-state momentum changes correct for the order variant Planck's constant over 2pi(2) errors due to the semiclassical approximation that are present even in single surface problems.

On the properties of a primitive semiclassical surface hopping propagator for nonadiabatic quantum dynamics.

A previously developed nonadiabatic semiclassical surface hopping propagator [M. F. Herman J.

On the importance of the classically forbidden region in calculations of the relaxation rate for high-frequency vibrations: a model calculation.

The quantum mechanical relaxation rate for a high-frequency vibrational mode is evaluated for a one-dimensional model system having two diatomic molecules involved in a collinear collision. The thermally averaged rate is obtained as an integral over energies for the relative translation of the two molecules. These calculations show that energies several times K(B)T make the largest contributions to the rate.

Using an r-dependent Gaussian width in calculations of the globally uniform semiclassical wave function.

The globally uniform semiclassical wave function expresses the solution to the time independent Schrodinger equation in terms of fixed width Gaussian wave packets traveling along a set of trajectories. There is a globally uniform wave function (GUWF) for each value of the Gaussian width parameter gamma. Numerical data show that a small Gaussian width is needed in some regions to obtain accurate results, while a broad Gaussian width provides better results in other regions.

A justification for a nonadiabatic surface hopping Herman-Kluk semiclassical initial value representation of the time evolution operator.

A justification is given for the validity of a nonadiabatic surface hopping Herman-Kluk (HK) semiclassical initial value representation (SC-IVR) method. The method is based on a propagator that combines the single surface HK SC-IVR method [J. Chem.

An analysis of the accuracy of an initial value representation surface hopping wave function in the interaction and asymptotic regions.

The behavior of an initial value representation surface hopping wave function is examined. Since this method is an initial value representation for the semiclassical solution of the time independent Schrodinger equation for nonadiabatic problems, it has computational advantages over the primitive surface hopping wave function. The primitive wave function has been shown to provide transition probabilities that accurately compare with quantum results for model problems.

Toward an accurate and efficient semiclassical surface hopping procedure for nonadiabatic problems.

The derivation of a semiclassical surface hopping procedure from a formally exact solution of the Schrodinger equation is discussed. The fact that the derivation proceeds from an exact solution guarantees that all phase terms are completely and accurately included. Numerical evidence shows the method to be highly accurate.

Nonadiabatic surface hopping Herman-Kluk semiclassical initial value representation method revisited: applications to Tully's three model systems.

The nonadiabatic surface hopping Herman-Kluk (HK) semiclassical initial value representation (SC-IVR) method for nonadiabatic problems is reformulated. The method has the same spirit as Tully's surface hopping technique [J. Chem.

Matching-pursuit/split-operator Fourier-transform simulations of nonadiabatic quantum dynamics.

A rigorous and practical approach for simulations of nonadiabatic quantum dynamics is introduced. The algorithm involves a natural extension of the matching-pursuitsplit-operator Fourier-transform (MPSOFT) method [Y. Wu and V.

Numerical study of the accuracy and efficiency of various approaches for Monte Carlo surface hopping calculations.

A one-dimensional, two-state model problem with two well-separated avoided crossing points is employed to test the efficiency and accuracy of a semiclassical surface hopping technique. The use of a one-dimensional model allows for the accurate numerical evaluation of both fully quantum-mechanical and semiclassical transition probabilities. The calculations demonstrate that the surface hopping procedure employed accounts for the interference between different hopping trajectories very well and provides highly accurate transition probabilities.

Globally uniform semiclassical surface-hopping wave function for nonadiabatic scattering.

A globally uniform time-independent semiclassical wave function for nonadiabatic scattering is presented. This wave function, which takes the form of a surface-hopping expansion, is motivated by the globally uniform semiclassical wave function of Kay and co-workers for the single-surface case. The surface-hopping expansion is similar to a previously presented primitive semiclassical wave function for nonadiabatic problems.