Hierarchy of equations to calculate the linear spectra of molecular aggregates: Time‐dependent and frequency domain formulation

In a recent publication (Ritschel et al., J. Chem. Phys. 2015, 142, 034115) we have derived a hierarchy of coupled differential equations in time domain to calculate the linear optical properties of molecular aggregates. Here, we provide details about issues concerning the numerical implementation. In addition we present the corresponding hierarchy in frequency domain. It is challenging to calculate vibronic absorption spectra of molecules that can exchange excitation via transition dipole–dipole interactions. One powerful approach is based on solving a hierarchy of differential equations in time domain. In this work, important aspects of an efficient implementation are discussed. In addition, a hierarchy directly in frequency domain is presented which provides a different viewpoint and also offers an alternative way to calculate linear spectra.