Year: 2026
Type: preprint
Abstract: Surface hopping is widely used to assign mechanisms in ultrafast molecular photodynamics, yet the resulting mechanistic picture can become strongly method-dependent when electronic coherence survives across recurrent nonadiabatic interactions. We identify this regime, which can arise in dense excited-state manifolds, as coherent recrossing. Using semiclassical analysis of the prototypical Tully dual avoided-crossing model, we disentangle three controlling ingredients: phase accumulation between nonadiabatic events, electronic decoherence, and the relative phase information encoded in the nonadiabatic coupling. We then examine [3.3]paracyclophane, comparing surface hopping with ab initio multiple spawning (AIMS) using the same Hamiltonian, initial conditions, and coupling treatment. AIMS predicts prolonged multistate population sharing with state-selective electronic coherence, whereas the tested surface-hopping variants yield dynamics qualitatively distinct from AIMS. Landau–Zener surface hopping (LZSH) does not propagate electronic amplitudes and consequently cannot retain phase memory between successive nonadiabatic events, preventing it from describing interference that depends on such memory. Fewest-switches surface hopping (FSSH) and local-diabatization surface hopping (LDSH), with and without standard decoherence correction, reveal a fourth controlling ingredient that emerges in the multistate regime: the state-pair selectivity of the retained coherence. The tested FSSH and LDSH treatments retain coherence less selectively than AIMS and fail to reproduce its population dynamics, ranging from rapid S1 funneling to excessively broad population sharing. However, because FSSH and LDSH propagate electronic amplitudes and therefore retain phase information, their description of coherent recrossing can in principle be improved through more appropriate treatments of decoherence.
Source: ChemRxiv
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7214704486