Q-Chem includes an “Ab Initio Frenkel-Davydov Exciton Model” that can calculate electron-phonon coupling between weakly coupled molecules. Can Q-Chem also calculate exciton-phonon coupling in the excited states of a molecule?

The derivative couplings of the AIFDEM are exciton/phonon couplings. See:

I have a few questions about the AIFDEM model in Q-Chem. I have only one molecule and am interested in Holstein (local) coupling. Do I need to run calculations with the original molecule and another with a displaced geometry? Additionally, Q-Chem is printing either the non-orthogonal AIFDEM Hamiltonian derivatives or the symmetric orthogonalization derivative in $QCSCRATCH/aifdem_deriv… How can I print the L matrix (Eqn. 45 of J. Chem. Phys. 146, 224110 (2017))?

AIFDEM is designed for intermolecular couplings. Usually we talk about the Holstein couplings as being the energy transfer integrals between different “sites”, which in the AIFDEM means different molecules. This should all be explained in the manual,

https://manual.q-chem.com/latest/Ch12.S13.SS1.html

To clarify, I have a single molecule and I want to calculate exciton-phonon coupling. AIFDEM does the calculation of intermolecular coupling. Are you saying that Q-Chem cannot calculate exciton-phonon coupling for a single molecule? I attempted to perform this calculation in Q-Chem but encountered an error.

I am not even sure what you mean by “exciton-phonon coupling of a single molecule”. can you point me to some definition in the literature?

I am looking something like this

If you’re referring to something like Eq. (12) in that paper, then if you strip away all of the wave vectors (for non-periodic calculation), the quantity $g^{s,\alpha}_{I,0}$ is fundamentally the nonadiabatic derivative coupling, which is available in Q-Chem at various levels of theory including TDDFT. “Electron-phonon coupling” is condensed matter physics language; Q-Chem mostly uses quantum chemistry language.

Thank you. I have one more question: Q-Chem is printing the non-orthogonal AIFDEM Hamiltonian derivatives in $QCSCRATCH/aifdem_deriv folder. How can I print the L matrix (Eqn. 45 of J. Chem. Phys. 146, 224110 (2017)) which is also required to calculate Holstein and Peierls coupling constants?

I don’t think the L matrix is part of the AIFDEM code. In the paper in question, we got it from phonon calculations in periodic DFT (in Quantum Espresso). In general, you get it from a vibrational frequency calculation of your choice; L is simply the matrix that transforms between Cartesian coordinates (in which the derivative couplings are calculated) and normal modes.