Rather general expressions are derived which represent the semiclassical time-dependent propagator as an integral over initial conditions for classical trajectories. These allow one to propagate time-dependent wave functions without searching for special trajectories that satisfy two-time boundary conditions. In many circumstances, the integral expressions are free of singularities and provide globally valid uniform asymptotic approximations. In special cases, the expressions for the propagators are related to existing semiclassical wave function propagation techniques. More generally, the present expressions suggest a large class of other, potentially useful methods. The behavior of the integral expressions in certain limiting cases is analyzed to obtain simple formulas for the Maslov index that may be used to compute the Van Vleck propagator in a variety of representations.

A semiclassical initial value technique for wave function propagation described by Herman and Kluk [Chem. Phys. 91, 27 (1984)] is tested for systems with two degrees of freedom. It is found that chaotic trajectories cause a serious deterioration in the accuracy and convergence of the technique. A simple procedure is developed to alleviate these difficulties, allowing one to propagate wave functions of a moderately chaotic system for relatively long times with good accuracy. This method is also applied to a very strongly chaotic system, the x2y2 or ‘‘quadric oscillator’’ model. The resulting energy spectra, obtained from the autocorrelation function of the wave function, are observed to be in good agreement with the corresponding quantal spectra. In addition, the density of states spectra, computed from the trace of the semiclassical propagator, are found to determine many individual energy levels of this system successfully.

We present numerical tests of five related semiclassical techniques for computing time-dependent wave functions. These methods are based on integral representations for the propagator and do not require searches for special trajectories satisfying double-ended boundary conditions. In many respects, the computational techniques involved resemble those of conventional quasiclassical treatments. Three of these methods result in globally uniform asymptotic approximations to the wave function. One such method, the treatment of Herman and Kluk, is found to be capable of especially high accuracy and rapid convergence.

This review describes some developments in the theory and application of the semiclassical initial representation for the treatment of the dynamical and static properties of atoms and molecules. The theoretical basis of initial value treatments for the propagator is discussed. A variety of useful alternative initial value expressions for the propagator and other quantities are presented as generalizations of the well-known Herman-Kluk approximation. Special emphasis is given to treatments that involve integration over only half the phase space variables. The recent development of semiclassical initial value expressions that are exact for specific, desired systems is reviewed and some of the implications are described.

Tunneling in the one-dimensional Eckart system is treated by a semiclassical method that describes the S-matrix in terms of an integral over the initial momenta of real-valued classical trajectories. The results are found to be sensitive to a certain parameter ␥ which is expected to be essentially arbitrary for classically allowed processes. Analysis of the semiclassical error allows formulation of conditions for the validity of the tunneling treatment. This, in turn, leads to an explanation for the sensitivity of the results to ␥ and an understanding of how this parameter should be chosen. With an optimized choice, the semiclassical method is found to yield very accurate tunneling results even for probabilities as small as 10 Ϫ10 . The relationship between the present method and the conventional uniform semiclassical treatment of barrier tunneling is discussed.