Publications

1990
Heller, E.J. Theoretical studies of time-of-flight and atom and molecular surface collision. (1990).
Heller, E.J. Mode mixing and chaos induced by potential surface crossings. The Journal of chemical physics 92, 1718–1727 (1990). Publisher's VersionAbstract

The effect of Born–Oppenheimer potential energy surface crossings on energy transfer in polyatomic molecules is investigated, classically and quantum mechanically. The hopping from one energy surface to another is enough to cause classical chaos, and strong mixing of the levels quantum mechanically. The r a t e of classical mixing determines the e x t e n t of quantum mixing, even though classical mixing is complete at long times.

1989
Huber, D., Ling, S., Imre, D.G. & Heller, E.J. Hybrid mechanics. II. J. Chem. Phys. 90, 7317 (1989). Publisher's VersionAbstract

We recently published a new method for the calculation of the time evolution of a wave function. We used an accurate approximate method to calculate the time propagator for a finite time Δt. Numerical calculations showed that this scheme works quite accurately, but that it is not more efficient than conventional methods. In this paper we propose to use a very fast and simple, but less accurate semiclassical method for the calculation of the time propagator. The approximation consists in the replacement of the Hamiltonian by a quadratic approximation around the center of the evolving wave packet called thawed Gaussian dynamics. We show by numerical examples in one and two dimensions that, despite this crude approximation, we achieve nearly the same accuracy as in the foregoing paper, but with an efficiency that is typically more than an order of magnitude better. We further show that the method is able to describe tunneling and long time dynamics (e.g., 1000 vibrational periods).

Henriksen, N.E. & Heller, E.J. Quantum dynamics for vibrational and rotational degrees of freedom using Gaussian wave packets: Application to the three-dimensional photodissociation dynamics of ICN. J. Chem. Phys. 91, 4700 (1989). Publisher's VersionAbstract

We present an approach to quantum dynamics, based entirely on Cartesian coordinates, which covers vibrational as well as rotational motion. The initial state is represented in terms of multidimensional Gaussian wave packets. Rotational adaptation to angular momentum eigenstates is done by using angular momentum projection operators. This gives an initial state represented as a weighted superposition of Gaussians with different average orientation in space. It is shown that the subsequent dynamics can be determined from the dynamics of Gaussians corresponding to just o n e of these orientations. An application to the 3Dphotodissociationdynamics of ICN is presented. All six degrees of freedom which describe the internal motion of the triatomic are included, the only approximation introduced in the present calculation being the thawed Gaussian wave packet approximation for the dynamics. The total absorptionspectrum out of vibrational–rotational eigenstates of ICN as well as fully resolved final product distributions are calculated.

1988
Huber, D., Heller, E.J. & Littlejohn, R.G. Generalized Gaussian wave packet dynamics, Schro¨dinger equation, and stationary phase approximation. J. Chem. Phys. 89, 2003 (1988). Publisher's VersionAbstract

We have recently published a new semiclassical method, generalized Gaussian wave packet dynamics, which extends Gaussian wave packet dynamics into complex phase space. Although we were able to give an accurate formulation of the method, we had at the time of writing that paper only an intuitive, heuristic understanding of the deeper causes which make the method work. A more mathematical understanding was needed. To close this gap we show in this paper the equivalence of the new method with a first order expansion of ℏ of the Schrödinger equation. We further prove that the new method is equivalent to the stationary phase approximation, using the usual WKB formula for the propagator. The latter equivalence enables us to show that all the symmetry properties of time‐dependent quantum mechanics also hold in the new semiclassical theory. Finally, we provide some elaboration of the method, and clarify several issues that were not discussed before. With this new insight we are able to formulate a simple rule for the calculation of semiclassical wave functions that contain contributions from more than one branch. This corrects for the divergence of semiclassical wave functions near caustics, a problem that we encountered in the preceding paper.

Frederick, J.H., Heller, E.J., Ozment, J.L. & Pratt, D.W. Ring torsional dynamics and spectroscopy of benzophenone: A new twist. J. Chem. Phys. 88, 2169 (1988). Publisher's VersionAbstract

The low energy portion of the high resolution S 1S 0fluorescence excitation spectrum of benzophenone recently reported by Holtzclaw and Pratt [J. Chem. Phys. 8 4, 4713 (1986)] is modeled here using a simple two‐degree‐of‐freedom vibrational Hamiltonian. The Hamiltonian features a 1:1 nonlinear resonance between the two low frequency ring torsional modes of the molecule in its S 1 state. Line positions and intensities of the two major spectral progressions are well reproduced using parameters similar to those derived from earlier matrix diagonalizations. The comparison of the theory and experiment results in a determination of the displacement of the S 1surface relative to the ground electronic state along the symmetric torsional coordinate and permits a calculation of the excitation spectra of various isotopically substituted molecules not yet measured in the laboratory. A clear picture of the relationship between the dynamics on the S 1surface and the spectroscopy of benzophenone is revealed by comparing a time domain analysis of the experimental data with wave packet dynamics on the model S 1surface. This comparison provides new insight into energy flow in the isolated molecule and permits a qualitative simulation of the effects of collisional quenching on the fluorescencespectrum. We also discuss, using a classical trajectory analysis, the resonance dynamics of the torsional modes and note the existence of heretofore undetected local modes in the high resolution spectrum.

1987
Heller, E.J., O'Connor, P.W. & Gehlen, J.N. Eigenfunctions and periodic orbits in classically chaotic systems: Order in chaos?. AIP Conference Proceedings 162, 43–59 (1987). Publisher's VersionAbstract

We study the quantum mechanics of a Hamiltonian system which is classically chaotic: the stadium billiard. We have examined many of the eigenstates of the stadium, up to about the 10,000th. Complex periodic orbits play an active role in shaping the eigenstates.

Frederick, J.H. & Heller, E.J. Multidimensional quantum eigenstates from the semiclassical dynamical basis set. J. Chem. Phys. 87, 6592 (1987). Publisher's VersionAbstract

 

A new method for obtaining molecular vibrational eigenstates using an efficient basis set made up of semiclassical eigenstates is presented. Basis functions are constructed from a ‘‘primitive’’ basis of Gaussian wave packets distributed uniformly on the phase space manifold defined by a single quasiperiodic classical trajectory (an invariant N‐torus). A uniform distribution is constructed by mapping a grid of points in the Hamilton–Jacobi angle variables, which parametrize the surface of the N‐torus, onto phase space by means of a careful Fourier analysis of the classical dynamics. These primitive Gaussians are contracted to form the semiclassical eigenstates via Fourier transform in a manner similar to that introduced by De Leon and Heller [J. Chem. Phys. 8 1, 5957 (1984)]. Since the semiclassical eigenstates represent an extremely good approximation of the quantum eigenstates, small matrix diagonalizations are sufficient to obtain eigenvalues ‘‘converged’’ to 4–5 significant figures. Such small diagonalizations need not include the ground vibrational state and thus can be used to find accurate eigenstates in select regions of the eigenvalue spectrum. Results for several multidimensional model Hamiltonians are presented.

 

Huber, D., Heller, E.J. & Harter, W.G. Semiclassical wave packet treatment of the rigid asymmetric rotor. J. Chem. Phys. 87, 1116 (1987). Publisher's VersionAbstract

We discuss a semiclassical treatment of the rigid asymmetric rotor that delivers eigenenergies as well as eigenstates. We give possibilities to improve the semiclassical wave functions to any accuracy required. The method is devised so that inclusion of vibrations is possible. As no information about energetically lower states is included in the procedure, the calculation of highly excited states is easier than with conventional quantum methods. Calculation of quantum splitting from semiclassical eigenstates is treated. We give numerical examples for every procedure developed, so that the performance of the theory can be judged.

1986
Parson, R.P. & Heller, E.J. Classical and semiclassical aspects of dissipative molecular processes: Radiative and nonradiative vibrational relaxation in polyatomic molecules. J. Chem. Phys. 85, 2581 (1986). Publisher's VersionAbstract

The time dependence of spontaneous emission from isolated, highly vibrationally excited molecules is studied from the point of view of the classical and semiclassical mechanics of dissipatively perturbed Hamiltonian systems. A correlation function expression for the time‐dependent emission spectrum serves as a starting point for quasiclassical approximations. These in turn lead to an investigation of the classical dynamics of model molecular systems in which small, energy‐nonconserving terms have been added to Hamilton’s equations. Numerical calculations show rich dynamical behavior which can be qualitatively understood in terms of the resonance structure of the unperturbed system. For example, trajectories tend to be captured by zones of nonlinear resonance. This ‘‘mode locking’’ produces a characteristic cleanup of an emission spectrum that otherwise is rather congested at these energies. The close connection between spectra and dynamics suggests that the classical dynamics of dissipatively perturbed systems may provide a useful language for interpreting radiative and condensed‐phase vibrational relaxation, and possibly some types of intramolecular relaxation phenomena as well.

Heller, E.J. Qualitative properties of eigenfunctions of classically chaotic Hamiltonian systems. Quantum Chaos and Statistical Nuclear Physics: Proceedings of the 2nd International Conference on Quantum Chaos and the 4th International Colloquium on Statistical Nuclear Physics, Held at Cuernavaca, México, January 6–10, 1986 162–181 (1986). Publisher's VersionAbstract

We begin by discussing the properties expected of eigenfunctions of a classically chaotic Hamiltonian system, using simple Correspondence Principle arguments. The properties involve nodal surfaces, coordinate and momentum space amplitude distribution, and phase space distribution. The eigenfunctions of the stadium billiard are examined, and it is found that the periodic orbits of shortest periods and smallest stability parameter profoundly affect the eigenfunctions: “scars” of higher wavefunction density surround the special periodic orbits. Finally a theory is presented for the scars, showing that they must exist, and relating them directly to the special periodic orbits. These same periodic orbits cause level density fluctuations.

Heller, E.J. & Reimers, J.R. Recent Progress in Semiclassical Wavepacket Methods for Chemical Physics Applications. Dynamics of Wave Packets in Molecular and Nuclear Physics: Proceedings of the International Meeting Held in Priorij Corsendonck, Belgium July 2–4, 1985 79-81 (1986). Publisher's Version
Parson, R.P. & Heller, E.J. Semiclassical mechanics of dissipative molecular processes: The correspondence principle and population relaxation. J. Chem. Phys. 85, 2569 (1986). Publisher's VersionAbstract

The relaxation of a primary system coupled weakly to a bath of environmental modes is examined from the standpoint of recent developments in the semiclassical theory of molecular bound states. Emphasis is placed upon highly excited, strongly nonlinear (but quasiperiodic) primary systems and zero temperature baths. The starting point for the analysis is a master equation for the populations of the eigenstates of the primary system. The correspondence principle provides semiclassical approximations to the transition rates, allowing quantum state populations to be calculated from classical trajectories. A second semiclassical approximation leads to an equation of motion for a probability density in the classical action variables. As h→0, this density agrees with the density generated by running an ensemble of damped classical trajectories and averaging out the angle variables; retention of terms of order h provides smoothed quantum corrections. Numerical examples of both semiclassical approximations are presented.

1985
Reimers, J.R., Wilson, K.R., Heller, E.J. & Langhoff, S.R. CASSCF-wave packet ab initio prediction of electronic and vibrational spectra: Application to the A(²Pi) reverse arrow X(². sigma. /sup +/) absorption of C2H at 3000 K. J. Chem. Phys. 82, 5064 (1985). Publisher's VersionAbstract

The combination of a b i n i t i o calculation of electronic wave functions with a wave packet calculation of the nuclear motion is used, within the Born–Oppenheimer approximation to compute the vibrational and electronic absorption of a polyatomic molecule. A particular virtue of this approach is that high as well as low temperature spectra are both calculable. This method is applied to C2H, for which the complete active space self‐consistent field (CASSCF) method is used to determine full Born–Oppenheimer potential surfaces. Using the assumption that the A(2Π) ← X(2Σ+) absorption can be written as the sum of the A(2A’) ← X and A(2A‘) ← Xabsorptions, the spectra are determined to 60 cm1 resolution at a temperature of 3000 K. As a result of the large thermal bending amplitude at 3000 K, the calculated spectra are broad and have little resolved structure. Two bands are resolvable, one is due to the A(2A‘) ← Xabsorption and is centered at 5500 cm1, while the other is due to A(2A’) ← Xabsorption and is centered at 9500 cm1. The dramatic blue shift of the A(2A’) ← X band results from the combination of the large X state thermal bending amplitude and high bending frequency of the A(2A’) state. We also determine the X state pure vibrational absorptionspectrum and show it to be of much lower intensity than the pure electronic spectrum.

Reimers, J.R. & Heller, E.J. The exact eigenfunctions and eigenvalues of a two-dimensional rigid rotor obtained using Gaussian wave packet dynamics. J. Chem. Phys. 83, 511 (1985). Publisher's VersionAbstract

Exact eigenfunctions for a two‐dimensional rigid rotor are obtained using Gaussian wave packet dynamics. The wave functions are obtained by propagating, without approximation, an infinite set of Gaussian wave packets that collectively have the correct periodicity, being coherent states appropriate to this rotational problem. This result leads to a numerical method for the semiclassical calculation of rovibrational, molecular eignestates. Also, a simple, almost classical, approximation to full wave packet dynamics is shown to give exact results: this leads to an a p o s t e r i o r i justification of the De Leon–Heller spectral quantization method.

Reimers, J.R. & Heller, E.J. The exact thermal rotational spectrum of a two-dimensional rigid rotor obtained using Gaussian wave packet dynamics. J. Chem. Phys. 83, 516, (1985). Publisher's VersionAbstract

 

The exact thermal rotational spectrum of a two‐dimensional rigid rotor is obtained using Gaussian wave packet dynamics. The spectrum is obtained by propagating, without approximation, infinite sets of Gaussian wave packets. These sets are constructed so that collectively they have the correct periodicity, and indeed, are coherent states appropriate to this problem. Also, simple, almost classical, approximations to full wave packet dynamics are shown to give results which are either exact or very nearly exact. Advantages of the use of Gaussian wave packet dynamics over conventional linear response theory are discussed.

 

Heller, E.J. & Sundberg, R.L. Quantum Ergodicity and Intensity Fluctuations. Chaotic Behavior in Quantum Systems 255-292 (1985). Publisher's VersionAbstract

Flow in phase space is intimately related to the concept of ergodicity. An ergodic system will sample all regions of phase space democratically in the time average, subject only to known a priori constants of the motion. For convenience, we shall refer to the accessible phase space as the “energy shell.” This shell may have some thickness if a distribution of energies are initially populated An energy envelope is defined by the initial distribution of energy and the envelope plays a role both in classical and quantum phase Space flow, as discussed before and as will become evident in the theory and examples to follow.

Reimers, J.R. & Heller, E.J. The exact thermal rotational spectrum of a two-dimensional rigid rotor obtained using Gaussian wave packet dynamics. Journal of Chemical Physics 83, 516 (1985). Publisher's VersionAbstract
The exact thermal rotational spectrum of a two-dimensional rigid rotor is obtained using Gaussian wave packet dynamics. The spectrum is obtained by propagating, without approximation, infinite sets of Gaussian wave packets. These sets are constructed so that collectively they have the correct periodicity, and indeed, are coherent states appropriate to this problem. Also, simple, almost classical, approximations to full wave packet dynamics are shown to give results which are either exact or very nearly exact. Advantages of the use of Gaussian wave packet dynamics over conventional linear response theory are discussed. [ABSTRACT FROM AUTHOR]
1984
Davis, M.J. & Heller, E.J. Comparisons of classical and quantum dynamics for initially localized states. J. Chem. Phys. 80, 5036 (1984). Publisher's VersionAbstract

We compare the dynamics of quantum wave packets with the dynamics of classical trajectory ensembles. The wave packets are Gaussian with expectation values of position and momenta which centers them in phase space. The classical trajectory ensembles are generated directly from the quantum wave packets via the Wigner transform. Quantum and classical dynamics are then compared using several quantum measures and the analogous classical ones derived from the Wigner equivalent formalism. Comparisons are made for several model potentials and it is found that there is generally excellent classical–quantum correspondence except for certain specific cases of tunneling and interference. In general, this correspondence is also very good in regions of phase space where there is classical chaos.

Eaker, C.W., Schatz, G.C., Leon, N.D. & Heller, E.J. Fourier transform methods for calculating action variables and semiclassical eigenvalues for coupled oscillator systems. J. Chem. Phys. 81, 5913 (1984). Publisher's VersionAbstract

Two methods for calculating the good action variables and semiclassical eigenvalues for coupled oscillator systems are presented, both of which relate the actions to the coefficients appearing in the Fourier representation of the normal coordinates and momenta. The two methods differ in that one is based on the exact expression for the actions together with the EBK semiclassical quantization condition while the other is derived from the Sorbie–Handy (SH) approximation to the actions. However, they are also very similar in that the actions in both methods are related to the same set of Fourier coefficients and both require determining the perturbed frequencies in calculating actions. These frequencies are also determined from the Fourier representations, which means that the actions in both methods are determined from information entirely contained in the Fourier expansion of the coordinates and momenta. We show how these expansions can very conveniently be obtained from fast Fourier transform (FFT) methods and that numerical filtering methods can be used to remove spurious Fourier components associated with the finite trajectory integration duration. In the case of the SH based method, we find that the use of filtering enables us to relax the usual periodicity requirement on the calculated trajectory. Application to two standard Henon–Heiles models is considered and both are shown to give semiclassical eigenvalues in good agreement with previous calculations for nondegenerate and 1:1 resonant systems. In comparing the two methods, we find that although the exact method is quite general in its ability to be used for systems exhibiting complex resonant behavior, it converges more slowly with increasing trajectory integration duration and is more sensitive to the algorithm for choosing perturbed frequencies than the SH based method. The SH based method is less straightforward to use in studying resonant systems, but good results are obtained for 1:1 resonant systems using actions defined in terms of the complex coordinates Q 1±i Q 2. The SH based method is also shown to be remarkably accurate in determining high energy eigenvalues (about three‐quarters of the dissociation energy).

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