Ab Initio Nonadiabatic Molecular Dynamics in Weakly Coupled Nanosystems

Nanoscale systems often contain weakly coupled components, as exemplified by layered materials. Time-domain atomistic modeling of excited state processes in such systems with nonadiabatic (NA) molecular dynamics (MD) runs into severe challenges due to the divergence of the NA coupling. At the same time, standard NAMD methods work well within each component. We develop an efficient ab initio NAMD methodology using a mixed diabatic-adiabatic representation (dNAMD), implement decoherence-induced surface hopping (DISH) within the dNAMD framework, and demonstrate its utility with long-range charge transfer in 2D perovskites taking place on nano- to microsecond time scales. The dNAMD method bypasses the trivial state crossing issue of traditional NAMD by using a diabatization technique to derive diabatic electronic coupling integrals between weakly coupled components, while employing adiabatic representation within each component. We demonstrate the approach by application to 2D perovskites, which are promising materials for optoelectronic applications, but show limited efficiencies because of the insulating nature of organic spacer cations and slow interlayer charge transport. The interlayer charge transfer time scales predicted by DISH-dNAMD are consistent with experimental data and Marcus rate constants. The simulations show that phenethylammonium spacers enhance inorganic lattice rigidity via strong hydrogen bonding and π-π stacking interactions, and reduce electron-vibrational coupling while increasing interlayer spacing and charge localization. These effects significantly reduce the electronic couplings, yielding charge transfer rates that are 1-2 orders of magnitude lower than those for the more structurally flexible butylammonium spacers. The DISH-dNAMD simulations highlight the critical role of the spacer rigidity in the interlayer charge transport of 2D perovskites. The developed dNAMD framework provides an efficient and versatile tool for simulating and elucidating excited state dynamics in weakly coupled nanoscale and condensed phase systems at the atomistic level and in the time domain as it occurs in nature and experiments, advancing the design of next-generation optoelectronic devices.