Multiconfigurational Gaussian wavepacket simulations of exciton diffusion in semiconducting polymer chains: Efficient finite-temperature simulations with Langevin driving

First-principles quantum-dynamical simulations of photoinduced exciton dynamics are carried out using the variational two-layer Gaussian-based multiconfiguration time-dependent Hartree (2L-GMCTDH) method combined with stochastic Langevin dynamics. Analogously to earlier reference calculations [Binder and Burghardt, Faraday Discuss. 221, 406 (2020)], a generalized Frenkel-Holstein Hamiltonian is constructed for a 20-site oligothiophene chain as a minimal model for intra-chain exciton migration in poly-(3-hexylthiophene) (P3HT). Here, exciton quasi-particles undergo polaronic trapping due to local high-frequency modes, while transport is induced by thermal driving due to ring-torsional modes. It is shown that the 2L-GMCTDH simulations provide a highly flexible and efficient framework where the use of multiple explicit local reservoirs can be replaced with multiple Langevin thermostats. The computation of temperature-dependent exciton diffusion coefficients is illustrated, along with the dependence on static disorder.