  We present an implementation of Triplet Excitation-Energy Transfer (TEET)
couplings based on subsystem-based Time-Dependent Density-Functional Theory
(sTDDFT). TEET couplings are systematically investigated by comparing "exact"
and approximate variants of sTDDFT. We demonstrate that, while sTDDFT utilizing
explicit approximate Non-Additive Kinetic Energy (NAKE) density functionals is
well-suited for describing Singlet Excitation-Energy Transfer (SEET) processes,
it is inadequate for characterizing TEET. However, we show that
Projection-based Embedding (PbE)-based sTDDFT addresses the challenges faced by
NAKE-sTDDFT and emerges as a promising method for accurately describing
electronic couplings in TEET processes.
  We also introduce the mixed PbE-/NAKE-embedding procedure to investigate TEET
effects in solvated pairs of chromophores. This approach offers a good balance
between accuracy and efficiency, enabling comprehensive studies of TEET
processes in complex environments.
