26 research outputs found
Interplay between Intra- and Intermolecular Charge Transfer in the Optical Excitations of J-Aggregates
In a first-principles study based on density functional theory and many-body
perturbation theory, we address the interplay between intra- and intermolecular
interactions in a J-aggregate formed by push-pull organic dyes by investigating
its electronic and optical properties. We find that the most intense excitation
dominating the spectral onset of the aggregate, i.e., the J-band, exhibits a
combination of intramolecular charge transfer, coming from the push-pull
character of the constituting dyes, and intermolecular charge transfer, due to
the dense molecular packing. We also show the presence of a pure intermolecular
charge-transfer excitation within the J-band, which is expected to play a
relevant role in the emission properties of the J-aggregate. Our results shed
light on the microscopic character of optical excitations of J-aggregates and
offer new perspectives to further understand the nature of collective
excitations in organic semiconductors.Comment: published under ACS Authorchoice licens
Quantifying the Plasmonic Character of Optical Excitations in a Molecular J-Aggregate
The definition of plasmon at the microscopic scale is far from being
understood. Yet, it is very important to recognize plasmonic features in
optical excitations, as they can inspire new applications and trigger new
discoveries by analogy with the rich phenomenology of metal nanoparticle
plasmons. Recently, the concepts of plasmonicity index and the generalized
plasmonicity index (GPI) have been devised as computational tools to quantify
the plasmonic nature of optical excitations. The question may arise whether any
strong absorption band, possibly with some sort of collective character in its
microscopic origin, shares the status of plasmon. Here we demonstrate that this
is not always the case, by considering a well-known class of systems
represented by J-aggregates molecular crystals, characterized by the intense J
band of absorption. By means of first-principles simulations, based on a
many-body perturbation theory formalism, we investigate the optical properties
of a J-aggregate made of push-pull organic dyes. We show that the effect of
aggregation is to lower the GPI associated with the J-band with respect to the
isolated dye one, which corresponds to a nonplasmonic character of the
electronic excitations. In order to rationalize our finding, we then propose a
simplified one-dimensional theoretical model of the J-aggregate. A useful
microscopic picture of what discriminates a collective molecular crystal
excitation from a plasmon is eventually obtained.Comment: Published by ACS under ACS AuthorChoice licens
Ionization Potentials of First-Row Transition Metal Aqua Ions
We report computations of the vertical ionization potentials within the
approximation of the near-complete series of first-row transition metal (V-Cu)
aqua ions in their most common oxidation states, i.e. V, Cr,
Cr, Mn, Fe, Fe, Co, Ni, and
Cu. The -orbital occupancy of these systems spans a broad range from
to . All the structures were first optimized at the density
functional theory level using a large cluster of explicit water molecules that
are embedded in a continuum solvation model. Vertical ionization potentials
were computed with the one-shot approach on a range of transition
metal ion clusters (6, 18, 40, and 60 explicit water molecules) wherein the
convergence with respect to the basis set size was evaluated using the systems
with 40 water molecules. We assess the results using three different density
functional approximations as starting points for the vertical ionization
potential calculations, namely @PBE, @PBE0, and
@rSCAN. While the predicted ground-state structures are similar
with all three exchange-correlation functionals, the vertical ionization
potentials were in closer agreement with the experiment when using the
@PBE0 and @rSCAN approaches, with the r2SCAN based
calculations being significantly less expensive. Computed bond distances and
vertical ionization potentials for all structures were compared with available
experimental data and are in good agreement
A Density-Based Basis-Set Incompleteness Correction for GW Methods
9 pages, 2 figures (supporting information available)International audienceSimilar to other electron correlation methods, many-body perturbation theory methods based on Green functions, such as the so-called approximation, suffer from the usual slow convergence of energetic properties with respect to the size of the one-electron basis set. This displeasing feature is due to lack of explicit electron-electron terms modeling the infamous Kato electron-electron cusp and the correlation Coulomb hole around it. Here, we propose a computationally efficient density-based basis set correction based on short-range correlation density functionals which significantly speeds up the convergence of energetics towards the complete basis set limit. The performance of this density-based correction is illustrated by computing the ionization potentials of the twenty smallest atoms and molecules of the GW100 test set at the perturbative (or ) level using increasingly large basis sets. We also compute the ionization potentials of the five canonical nucleobases (adenine, cytosine, thymine, guanine, and uracil) and show that, here again, a significant improvement is obtained
Assessment of the Ab Initio Bethe-Salpeter Equation Approach for the Low-Lying Excitation Energies of Bacteriochlorophylls and Chlorophylls
Bacteriochlorophyll and Chlorophyll molecules are crucial building blocks of
the photosynthetic apparatus in bacteria, algae and plants. Embedded in
transmembrane protein complexes, they are responsible for the primary processes
of photosynthesis: excitation energy and charge transfer. Here, we use ab
initio many body perturbation theory within the approximation and
Bethe-Salpeter equation (BSE) approach to calculate the electronic structure
and optical excitations of Bacteriochlorophylls , , , and and
Chlorophylls and . We systematically study the effects of structure,
basis set size, partial self-consistency in , and the underlying
exchange-correlation approximation, and compare our calculations with results
from time-dependent density functional theory, multireference RASPT2 and
experimental literature results. We find that optical excitations calculated
with +BSE are in excellent agreement with experimental data, with an
average deviation of less than 100\,meV for the first three bright excitations
of the entire family of (Bacterio)chlorophylls. Contrary to state-of-the-art
TDDFT with an optimally-tuned range-separated hybrid functional, this accuracy
is achieved in a parameter-free approach. Moreover, +BSE predicts the
energy differences between the low-energy excitations correctly, and eliminates
spurious charge transfer states that TDDFT with (semi)local approximations is
known to produce. Our study provides accurate reference results and highlights
the potential of the +BSE approach for the simulation of larger pigment
complexes