An Efficient N(3)-Scaling Propagation Scheme for Simulating Two-Dimensional Infrared and Visible Spectra.

In this paper, we develop and test a new approximate propagation scheme for calculating two-dimensional infrared and visible spectra. The new scheme scales one order more efficiently with the system size than the existing schemes. A Trotter type of approximation is used for the matrix exponent that describes the time evolution of the quantum system. This is needed for calculating the third-order response functions governing the two-dimensional infrared and visible spectra. The method is tested on a model dimer system, the amide I spectrum of the Gramicidin A antimicrobial peptide, the spectrum of the OH stretching vibration of bulk water, and a homogeneous J-aggregate. Errors due to the approximation are hardly observable in the calculated spectra. Scaling simulations with different system sizes are used to demonstrate that the new scheme is indeed scaling with the system size to the third power, one order faster than the existing methods.