Simulation of absorption spectra of molecular aggregates: a Hierarchy of Stochastic Pure States approach

The simulation of spectroscopic observables for molecular aggregates with strong and structured coupling of electronic excitation to vibrational degrees of freedom is an important but challenging task. The hierarchy of pure states (HOPS) provides a formally exact solution based on local, stochastic trajectories. Exploiting the localization of HOPS for the simulation of absorption spectra in large aggregares requires a formulation in terms of normalized trajectories. Here we provide a normalized dyadic equation where the ket- and bra-states are propagated in different electronic Hilbert spaces. This work opens the door to apply adaptive HOPS methods for the simulation of absorption spectra and also to a formulation for non-linear spectroscopy that is perturbative with respect to interactions with the electric field.