Mixed quantum-classical simulations of intramolecular singlet fission

Intramolecular singlet fission is a multistate nonadiabatic process in which vibronic couplings govern ultrafast triplet-pair formation and subsequent relaxation. We benchmark mixed quantum-classical approaches-Ehrenfest dynamics, linearized semiclassical spin-mapping (spin-LSC), and the map-ping approach to surface hopping (MASH) - against numerically exact multilayer multicon-figuration time-dependent Hartree (ML-MCTDH) calculations for models of phenylene-linked pentacene dimers. While Ehrenfest dynamics exhibits pronounced mean-field artifacts and incorrect long-time populations, spin-LSC improves short-time transfer but suffers from transient negative populations and back-transfer effects. MASH provides the most consistent agreement across timescales and molecular variants and remains free of unphysical populations. We further analyze the role of zero-point motion in the initial vibrational distribution and show that it affects the accuracy of different mapping-based approaches in distinct ways. Our results demonstrate that spin-mapping surface-hopping methods offer a robust route to extend singlet-fission simulations beyond the regime accessible to numerically exact quantum dynamics.