Computational Studies of Excited States of Chlorophyll Dimers in Light-Harvesting Complexes
Authors:   , Sen Souloke 
Published: January 2023

Light harvesting in higher plants involves a complex multi-dimensional problem
the solution of which encompasses contributions from various fields of research
involving experiments and theoretical modelling. The intricate relation between
the structure and spectral properties of such complexes lies at the
core of its diverse functionality and had been brought to light many times in
the past, both experimentally and theoretically. A broader understanding of the
interplay between the EET and ET process which make up the complex energy
landscape of such complexes becomes crucial when attempting to elucidate such
relationships.

A proper combination of the QM or QM/MM
method(s) with structures generated from a long classically simulated MD
trajectory provides an efficient strategy of including dynamical effects of
the protein-pigment complex on the computed quantities of interest. Such
incorporation of long time scale conformational changes is essential when
elucidating the structure-property relation of the complexes. The additional spectral changes that accompany such conformational changes and which were oblivious in the original work are reported in chapters 3 and 4. In particular, we observed two prominent spectral changes
that are discussed in chapters 3 and 4 respectively - 1) A drop in the oscillator
strength of the chlorophyll dimer which resulted from an increase in the charge
transfer contribution of the total oscillator strength, and 2) An accompanying
red-shift of the lowest excitonic state on account of mixing with charge transfer
states which drops in energy for the equilibrated structure of the two chlorophylls
towards the end of the MD trajectory.

In chapter 5 we presented a generalized approach wherein molecular fragment
orbitals could be used as a reference basis to generate a set of localized occupied
and valence virtual orbitals (denoted as ILMOs). Since the original localization
scheme of obtaining ILMOs as described in chapter 2 retained only a minimal
description of the virtual valence space, we further explored a pragmatic approach
for localizing higher-lying hard virtual orbitals using a similar procedure as done
for the occupied orbitals. The ILMOs, by themselves, are an interesting set of
molecular orbitals which upon visualization can already provide a qualitative
understanding of the nature of chemical bonding and antibonding orbitals as
demonstrated. Since they represent an orthogonal set of orbitals, it can also be
directly used in subsequent embedding calculations without the need for the
construction of any projection operators. In this regard, we presented a simple
one-step embedding approach for generating a set of re-canonicalized ILMOs
(RILMOs) for each subsystem starting from a set of CMOs. In contrast to the
ILMOs, these orbitals are delocalized over individual subsystem and retain the
intrinsic π and π∗ character corresponding to bonding and antibonding orbitals
as encountered in isolated conjugated systems. These orbitals are comparable to a
set of polarized orthonormal orbitals arising from a top-down projection based
embedding procedure. Moreover, in this chapter, we outlined a strategy for obtaining LE and CT approximate diabatic states from TDDFT including their corresponding electronic couplings in the basis of these orbitals. We also investigated the effect
of different levels of truncation (truncation in the number of hard virtuals kept
in the localization procedure versus truncation in the number of diabatic states
used to reproduce the exact supersystem results) on the energies of the final
adiabatic states. Such truncations are in the best interest of studying excited state
processes in medium to large-sized molecules where working with matrices of
reduced dimension can be computationally beneficial. Overall, our framework
is general and can be easily extended to any other excited state methods which is envisaged as part of future work.