An All-DFTB Approach to the Parametrization of the System-Bath Hamiltonian Describing Exciton-Vibrational Dynamics of Molecular Assemblies

TL;DR

This paper introduces an all-DFTB method for accurately determining spectral densities essential for simulating exciton-vibrational dynamics in molecular assemblies, ensuring consistency in electronic structure and dynamics.

Contribution

The authors develop a unified all-DFTB protocol to compute spectral densities for Frenkel excitons, integrating electronic structure and vibrational effects consistently.

Findings

Successfully applied to a PTCDI crystal model.

Accurate calculation of monomer excitation energies and Coulomb couplings.

Effective mapping onto harmonic oscillator spectral densities.

Abstract

Spectral density functions are central to the simulation of complex many body systems. Their determination requires to make approximations not only to the dynamics but also to the underlying electronic structure theory. Here, blending different methods bears the danger of an inconsistent description. To solve this issue we propose an all-DFTB approach to determine spectral densities for the description of Frenkel excitons in molecular assemblies. The protocol is illustrated for a model of a PTCDI crystal, which involves the calculation of monomeric excitation energies, Coulomb couplings between monomer transitions, as well as their spectral distributions due to thermal fluctuations of the nuclei. Using dynamically defined normal modes, a mapping onto the standard harmonic oscillator spectral densities is achieved.