DRAGON: A Versatile Open-Source Software for Diabatization of Multiple Excited States beyond Two Fragments

The diabatic representation provides a natural framework for describing relevant electronic processes in molecular materials (e.g., optical properties, exciton transport, exciton dissociation, singlet fission, energy transfer between radicals…) using states with a well-defined character (diabatic states) and the electronic couplings between them. This motivates the development of general diabatization methods to generate these diabatic states and couplings. Herein, we present DRAGON, an open-source software for the diabatization of multiple excited states in molecular aggregates beyond two fragments. DRAGON implements the fragment particle–hole density (FPHD) diabatization scheme of Wang and coworkers (J. Phys. Chem. Lett. 2021, 12, 1032–1039; J. Chem. Theory Comput. 2023, 19, 3900–3914) and interfaces it with widely used quantum chemistry packages, including Gaussian 16, ORCA, PySCF, Psi4, and xTB. Our implementation provides an extended version of FPHD, validated against reference results, that broadens the application limits in three directions with respect to the original formulation. DRAGON can compute diabatic states of different multiplicity (e.g., singlet and triplet) coupled by spin–orbit coupling (SOC), can tackle unrestricted wavefunctions enabling the diabatization of multiple excited states in molecular aggregates of open-shell systems (e.g., stable organic radicals), and can give an efficient route to obtain massive diabatic states and couplings in large-size molecular aggregates or materials owing to the coupling between FPHD with the Grimme's simplified Tamm–Dancoff approximation (sTDA). We show that the latter extension is interesting to take into account environmental effects beyond the nearest-neighbor approximation during the diabatization process.