Response functions from Fourier component variational perturbation theory applied to a time-averaged quasienergy
✍ Scribed by Ove Christiansen; Poul Jørgensen; Christof Hättig
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 534 KB
- Volume
- 68
- Category
- Article
- ISSN
- 0020-7608
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✦ Synopsis
It is demonstrated that frequency-dependent response functions can conveniently be derived from the time-averaged quasienergy. The variational criteria for the quasienergy determines the time-evolution of the wave-function parameters and the time-averaged time-dependent Hellmann᎐Feynman theorem allows an identification of response functions as derivatives of the quasienergy. The quasienergy therefore plays the same role as the usual energy in time-independent theory, and the same techniques can be used to obtain computationally tractable expressions for response properties, as for energy derivatives in time-independent theory. This includes the use of the variational Lagrangian technique for obtaining expressions for molecular properties in accord with the 2 n q 1 and 2 n q 2 rules. The derivation of frequency-dependent response properties becomes a simple extension of variational perturbation theory to a Fourier component variational perturbation theory. The generality and simplicity of this approach are illustrated by derivation of linear and higher-order response functions for both exact and approximate wave functions and for both variational and nonvariational wave functions. Examples of approximate models discussed in this article are coupled-cluster, selfconsistent field, and second-order Møller᎐Plesset perturbation theory. A discussion of symmetry properties of the response functions and their relation to molecular properties is also given, with special attention to the calculation of transition-and excited-state properties.