Fourier transform methods for calculating action variables and semiclassical eigenvalues for coupled oscillator systems

Charles W. Eaker, George C Schatz, Nelson De Leon, Eric J. Heller

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55 Citations (Scopus)

Abstract

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 Q1±iQ2. The SH based method is also shown to be remarkably accurate in determining high energy eigenvalues (about three-quarters of the dissociation energy).

Original languageEnglish
Pages (from-to)5913-5919
Number of pages7
JournalJournal of Chemical Physics
Volume81
Issue number12
Publication statusPublished - 1984

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Fourier transforms
eigenvalues
Trajectories
oscillators
trajectories
Momentum
momentum
expansion
coefficients
complex systems
Fast Fourier transforms
periodic variations
dissociation
requirements
energy
approximation

ASJC Scopus subject areas

  • Atomic and Molecular Physics, and Optics

Cite this

Fourier transform methods for calculating action variables and semiclassical eigenvalues for coupled oscillator systems. / Eaker, Charles W.; Schatz, George C; De Leon, Nelson; Heller, Eric J.

In: Journal of Chemical Physics, Vol. 81, No. 12, 1984, p. 5913-5919.

Research output: Contribution to journalArticle

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