On algorithms for numerical calculation of absoption spectra of nonequilibrium molecular systems

This study relates to computational aspects of computer simulations of ultrafast photochemical reactions in donor-acceptor molecular systems, placed in a polar solvent. The emphasis is put on development of algorithms, that can be employed for calculations of transient absorption spectra, detected on the molecular system in the course of photochemical reaction. These algorithms can be used both for the analysis of experimental data, received by means of femtosecond pump-probe spectroscopy, and for the numerical studies on kinetics of ultrafast chemical reactions in nonequilibrium molecular systems.
Within the resonant approximation, analytic expressions for the absorption spectrum of a molecular system interacting with a short laser probe pulse are derived. Two particular cases — without any spectral broadening and with the gaussian line shape — are considered. Computational schemes, compartible with the recrossing algorithms for multistage photochemical reactions simulations, are proposed for each of these cases. Computational complexities of the corresponding schemes are estimated.
It is shown, that the efficiency of calculations can be considerably increased by means of simple optimization procedures. These procedures involve precalculation and storing in RAM the spectral profiles of the donor-acceptor system along the solvent energetic coordinate. The effect of such optimization is estimated for typical molecular systems.