  We introduce a novel methodology for simulating the excited-state dynamics of
extensive molecular aggregates in the framework of the long-range corrected
time-dependent density-functional tight-binding fragment molecular orbital
method (FMO-LC-TDDFTB) combined with the mean-field Ehrenfest method. The
electronic structure of the system is described in a quasi-diabatic basis
composed of locally excited and charge-transfer states of all fragments. In
order to carry out nonadiabatic molecular dynamics simulatios, we derive and
implement the excited-state gradients of the locally excited and
charge-transfer states. Subsequently, the accuracy of the analytical
excited-state gradients is evaluated. The applicability to the simulation of
exciton transport in organic semiconductors is illustrated on a large cluster
of anthracene molecules. Additionally, nonadiabatic molecular dynamics
simulations of a model system of benzothieno-benzothiophene molecules highlight
the methods utility in studying charge-transfer dynamics in organic materials.
Our new methodology will facilitate the investigation of excitonic transfer in
extensive biological systems, nanomaterials and other complex molecular systems
consisting of thousands of atoms.
