The minimal model of light harvesting complex with dipole-quadrupole interaction derived from ab initio calculation

TL;DR

This paper introduces a minimal excitonic Hamiltonian model for purple bacteria's light harvesting complex, incorporating ab initio calculated dipole and quadrupole moments, revealing how dipole-quadrupole interactions influence spectral properties and subsystem separation.

Contribution

A novel one-parameter model derived from ab initio data that includes dipole-quadrupole interactions to better understand energy transport in light harvesting complexes.

Findings

Dipole-quadrupole interactions cause near-complete separation of B800 and B850 subsystems.

Spectral changes due to these interactions are small but significant for system analysis.

The model simplifies understanding of energy transport pathways in LH2 complexes.

Abstract

A minimal, one-parameter model of the excitonic Hamiltonian for the light harvesting complex of purple bacteria (LH2) based on ab initio calculation of the excitonic energies is proposed. The set of input parameters contains positions of atoms in 27 bacteriochlorophyll pigments only. The excitation energies, transition dipole and quadrupole moments of the bacteriochlorophyll units were calculated by advanced multiconfigurational multireference method. In the work we investigate influence of the dipole-quadrupole interaction on the particularities of the absorption spectra and circular dichroism and on the wave-functions localization. We demonstrate that although the spectra changes are small the dipole-quadrupole interaction term leads, however, to almost complete separation of the system onto two subsystems: the small B800 ring of pigments and the large B850 ring. The latter observation is seemed essential for formulation of energy transport models in light harvesting complexes.