A multiconfigurational wavefunction implementation of the Frenkel exciton model for molecular aggregates

We present an implementation of the Frenkel exciton model into the OpenMolcas program package enabling calculations of collective electronic excited states of molecular aggregates based on a multireference wavefunction description of the individual monomers. The computational protocol avoids using diabatization schemes and, thus, inherent supermolecule calculations. Additionally, the employment of the Cholesky decomposition of the aggregate's two-electron integrals enhances the efficiency of the computational scheme. The application of the method is exemplified for two test systems, that is, a formaldehyde oxime and a bacteriochlorophyll dimer. For the sake of comparison with the dipole approximation we restrict our considerations to situations where intermonomer exchange can be neglected.The protocol is expected to be beneficial for molecules with extended $\pi$ systems, unpaired electrons such as radicals or transition metal centers, where it should outperform widely-used methods based on time-dependent density functional theory.