Excitonic Configuration Interaction: Going Beyond the Frenkel Exciton Model

We present the excitonic configuration interaction (ECI)
methoda
fragment-based analogue of the CI method for electronic structure
calculations of multichromophoric systems. It can also be viewed as
a generalization of the exciton approach, with the following properties:
(i) It constructs the effective Hamiltonian exclusively from monomer
calculations. (ii) It employs the strong orthogonality assumption
and is exact within McWeeny’s group function theory, thus requiring
only one-electron density matrices of the monomer states. (iii) It
is agnostic of the monomer electronic structure method, allowing us
to use/combine different methods. (iv) It includes an embedding point
charge scheme (called excitonic Hartree–Fock, EHF) to improve
the accuracy of the monomer states, but such that the effective full-system
Hamiltonian is not explicitly dependent on the embedding. (v) It is
systematically improvable, by expanding the set of monomer states
and by including configurations where two or more monomers are excited
(defining the ECIS, ECISD, etc., methods). The performance of ECI
is assessed by computing the absorption spectrum of two exemplary
multichromophoric systems, using CIS as the monomer electronic structure
method. The accuracy of ECI significantly depends on the chosen embedding
charges and the ECI expansion. The most accurate assessed combinationsECIS
or ECISD with EHF embeddingyield spectra that agree qualitatively
and quantitatively with full-system direct calculations, with deviations
of the excitation energies below 0.1 eV. We also show that ECISD based
on CIS monomer calculations can predict states where two monomers
are excited simultaneously (e.g., triplet–triplet double-local
excitations) that are inaccessible in a full-system CIS calculation.