Low-Scaling Quantum Chemistry Approach to Excited-State Properties via an ab Initio Exciton Model: Application to Excitation Energy Transfer in a Self-Assembled Nanotube

We introduce a charge-embedding scheme
for an excited-state quantum
chemistry method aimed at weakly interacting molecular aggregates.
The Hamiltonian matrix for the aggregate is constructed in a basis
of direct products of configuration-state functions for the monomers,
and diagonalization of this matrix affords excitation energies within
∼0.2 eV of the corresponding supersystem calculation. Both
the basis states and the coupling matrix elements can be computed
in a distributed way, resulting in an algorithm whose time-to-solution
is independent of the number of chromophores, and we report calculations
on systems with almost 55 000 basis functions using fewer than
450 processors. In a semiconducting organic nanotube, we find evidence
of ultrafast, coherent dynamics followed by energy localization driven
by static disorder. Truncation of the model system has a qualitative
effect on the energy-transfer dynamics, demonstrating the importance
of simulating an extended portion of the nanotube, which is not feasible
using traditional quantum chemistry.