THE SIMULATION OF ELECTRONIC ABSORPTION-SPECTRUM OF A CHROMOPHORE COUPLED TO A CONDENSED-PHASE ENVIRONMENT - MAXIMUM-ENTROPY VERSUS SINGULAR-VALUE DECOMPOSITION APPROACHES

We consider the problem of calculating the electronic absorption spect rum of a chromophore with intramolecular degrees of freedom coupled to a condensed phase environment. We approach this calculation in the fr amework of the imaginary-time path integral Monte Carlo techniques, an d focus on the problem of the analytic continuation of the imaginary-t ime data to the real-time axis. Two alternative analytic continuation methods are considered: the maximum entropy method and the singular va lue decomposition method. An exactly solvable model is introduced to t est the accuracy of these methods. Exact numerical results for the abs orption spectra are compared to the spectra reconstructed by the analy tic continuation methods; it is found that the singular value decompos ition method gives systematically higher resolution than the maximum e ntropy method and is capable of reproducing the fine vibronic structur e of the absorption spectrum. (C) 1997 American Institute of Physics.