Second derivative couplings are neglected completely. This appears on its face to be an approximation but has actually been shown to accurately account for errors arising from unphysical discontinuities in the adiabatic electronic wave functions. The implementations of AIMS in PySpawn uses a task-based approach (Figure ), leveraging an asynchronous propagation of the classical and quantum variables. This approach recognizes that AIMS simulations involve three hierarchical processes: creating a timedependent basis of TBFs through classical propagation and spawning, performing the required electronic structure calculations at centroid geometries to build the Hamiltonian, and prop- and extended dynamically weighted (XDW-CASPT2 ). Since its development, MS-calculated at the Franck-Condon region of the simulation of static absorption spectra. 61