  Diabatic states and the couplings between them are important for quantifying,
elucidating, and predicting the rates and mechanisms of many chemical and
biochemical processes. Here, we propose and investigate approaches to
accurately compute diabatic couplings from density functional theory (DFT)
using absolutely localized molecular orbitals (ALMOs). ALMOs provide an
appealing approach to generate variationally optimized diabatic states and
obtain their associated forces that allows for the relaxation of the donor and
acceptor orbitals in a way that is internally consistent in how the method
treats both the donor and acceptor states. Here, we show that one can obtain
more accurate electronic couplings between ALMO-based diabats by employing the
symmetrized transition density matrix to evaluate the exchange-correlation
contribution. We demonstrate that this approach yields accurate results in
comparison to other commonly used DFT-based diabatization methods across a wide
array of electron and hole transfer processes occurring in systems ranging from
conjugated organic molecules, such as thiophene and pentacene, to DNA base
pairs. We also show that this approach yields accurate diabatic couplings even
when combined with lower tiers of the DFT hierarchy, opening the door to
combining it with quantum dynamics approaches to provide an ab initio treatment
of nonadiabatic processes in the condensed phase.
