  Quantum--Mechanical methods that are both computationally fast and accurate
are not yet available for electronic excitations having charge transfer
character. In this work, we present a significant step forward towards this
goal for those charge transfer excitations that take place between
non-covalently bound molecules. In particular, we present a method that scales
linearly with the number of non-covalently bound molecules in the system and is
based on a two-pronged approach: The molecular electronic structure of
broken-symmetry charge-localized states is obtained with the Frozen Density
Embedding formulation of subsystem Density-Functional Theory; subsequently, in
a post-SCF calculation, the full-electron Hamiltonian and overlap matrix
elements among the charge-localized states are evaluated with an algorithm
which takes full advantage of the subsystem DFT density partitioning technique.
The method is benchmarked against Coupled-Cluster calculations and achieves
chemical accuracy for the systems considered for intermolecular separations
ranging from hydrogen-bond distances to tents of {\AA}ngstroms. Numerical
examples are provided for molecular clusters comprised of up to 56
non-covalently bound molecules.
