Éric Brémond

Visualizing and Quantifying Interactions in the Excited State

Determining the location and nature of the electron pairs within a molecule provides an intuitive representation of electronic structures. Yet, most of the available theoretical representations are not suitable for describing excited state phenomena. The density overlap region indicator (DORI) scalar field, which depends only on the density and its derivatives, overcomes previous limitations, while keeping the intuitiveness of popular scalar fields. Here, its usefulness is demonstrated by pinpointing visual and numerical DORI signatures for both intra- and intermolecular excited state situations.

Seeking for parameter-free double-hybrid functionals: The PBE0-DH model

A new double hybrid functional is herein presented, in which the ratio of the Hartree-Fock, Kohn-Sham, and perturbation terms (MP2) is determined on the basis of physical considerations. This functional, denominated PBE0 Double Hybrid (PBE0-DH), contains a small MP2 contribution (12.5%) and it has been tested on a number of different molecular properties, including atomization energies, weak interactions, and reaction energies. From the obtained results, PBE0-DH seems to represent a significant improvement with respect to the parent PBE0 functional, a parameter-free hybrid. Overall its performances are comparable to the ones found using other parameterized double hybrids. From a more general point of view, our work points out that, already showed for classes of other functionals, reliable and general-purpose double hybrids can be obtained on purely theoretical bases, without the use of any empirical fitting.

TD-DFT Vibronic Couplings in Anthraquinones: From Basis Set and Functional Benchmarks to Applications for Industrial Dyes.

Vibrationally resolved absorption spectra of a series of anthraquinoidic dyes have been obtained with a polarizable continuum model time-dependent density functional theory approach. Firstly, we assessed the impact of the atomic basis set on both the transition energies and the vibronic shapes of 1,4-NH2-anthraquinone using a large panel of Poples basis sets, up to the 6-311++G(3df,3pd). In a second stage, an extensive functional benchmark has been performed to determine an adequate approach for the same compound. In the third step, a complete analysis of the origin of the band shape was performed for the same derivative. In the fourth stage, a set of functionals has been applied to investigate the position isomers in the dihydroxy anthraquinone series. Finally, in a last phase, the methodology has been used for three dyes of technological interest. It turns out that the chosen basis set has a relatively limited impact on the computed transition energies as well as the topology of the vibronic shape, but both are significantly influenced by the selected functional. In the present case, no single functional simultaneously provides highly accurate positions and intensities of the different bands, but ωB97XD appears to be a good compromise. This analysis allows to rationalize the difference in shapes experimentally noticed for the visible band of apparently similar anthraquinones.

Doubly hybrid density functional xDH-PBE0 from a parameter-free global hybrid model PBE0

Following the XYG3 model which uses orbitals and density from B3LYP, an empirical doubly hybrid (DH) functional is developed by using inputs from PBE0. This new functional, named xDH-PBE0, has been tested on a number of different molecular properties, including atomization energies, bond dissociation enthalpies, reaction barrier heights, and nonbonded interactions. From the results obtained, xDH-PBE0 not only displays a significant improvement with respect to the parent PBE0, but also shows a performance that is comparable to XYG3. Arguably, while PBE0 is a parameter-free global hybrid (GH) functional, the B3LYP GH functional contains eight fit parameters. From a more general point of view, the present work points out that reliable and general-purpose DHs can be obtained with a limited number of fit parameters.

Communication: Rationale for a new class of double-hybrid approximations in density-functional theory

We provide a rationale for a new class of double-hybrid approximations introduced by Brémond and Adamo [J. Chem. Phys. 135, 024106 (2011)] which combine an exchange-correlation density functional with Hartree-Fock exchange weighted by λ and second-order Møller-Plesset (MP2) correlation weighted by λ(3). We show that this double-hybrid model can be understood in the context of the density-scaled double-hybrid model proposed by Sharkas et al. [J. Chem. Phys. 134, 064113 (2011)], as approximating the density-scaled correlation functional E(c)[n(1/λ)] by a linear function of λ, interpolating between MP2 at λ = 0 and a density-functional approximation at λ = 1. Numerical results obtained with the Perdew-Burke-Ernzerhof density functional confirms the relevance of this double-hybrid model.

Communication: Double-hybrid functionals from adiabatic-connection: The QIDH model

A new approach stemming from the adiabatic-connection (AC) formalism is proposed to derive parameter-free double-hybrid (DH) exchange-correlation functionals. It is based on a quadratic form that models the integrand of the coupling parameter, whose components are chosen to satisfy several well-known limiting conditions. Its integration leads to DHs containing a single parameter controlling the amount of exact exchange, which is determined by requiring it to depend on the weight of the MP2 correlation contribution. Two new parameter-free DHs functionals are derived in this way, by incorporating the non-empirical PBE and TPSS functionals in the underlying expression. Their extensive testing using the GMTKN30 benchmark indicates that they are in competition with state-of-the-art DHs, yet providing much better self-interaction errors and opening a new avenue towards the design of accurate double-hybrid exchange-correlation functionals departing from the AC integrand.

Impact of Vibronic Couplings on Perceived Colors: Two Anthraquinones as a Working Example.

The accurate simulation of the dye colors remains a significant challenge for theoreticians, notably due to the sensitivity of the human eyes that are able to distinguish variations of hues corresponding to trifling energetic shifts. Using time-dependent density functional theory and two hybrid functionals, we have simulated vibrationally resolved absorption spectra of two anthraquinone derivatives (solvent blue 35 and solvent green 3) solvated in cyclohexane. Comparisons with recent experiments demonstrate the efficiency of the ωB97X-D/6-31++G(d,p) approach for these structures. The impact of microscopic vibronic couplings on the macroscopic chromatic coordinates of the dyes is quantified. This work unravels the key role of these couplings and is consequently a step further in the modeling of human-perceived colors from purely ab initio models.

DFT and Proton Transfer Reactions: A Benchmark Study on Structure and Kinetics

A significant number of different exchange correlation functionals, ranging from generalized gradient approximations to double hybrids, has been tested on a difficult playground represented by proton transfer reactions. In order to have a complete picture of their performances, both energetics and structural features have been compared and the obtained ranking compared with those issued from the standard test for kinetics (i.e., the DBH24/08 set). Among all of the functionals, the ωB97X, BMK, B1LYP, and PBE0-DH approaches are those providing a good error balance on all four trials. Beyond these figures, the obtained results allow for some general considerations, such as those on the role of Hartree-Fock exchange in reaction barriers or the relation between structure and energetics.

Communication: One third: A new recipe for the PBE0 paradigm

We analyze the performances of the parameter-free hybrid density functional PBE0-1/3 obtained combining the PBE generalized-gradient functional with a predefined amount of exact exchange of 1/3, as recently discussed by Cortona [J. Chem. Phys. 136, 086101 (2012)]. The numerical results that we have obtained for various properties, such as atomization energies (G2-148 dataset), weak interactions (NCB31 dataset), hydrogen-bond length optimizations, and dissociation energies (HB10 dataset), and vertical excitation energies, show an increased performance of PBE0-1/3 with respect to the widely used PBE0. We therefore propose to use one third as the mixing coefficient for the PBE-based hybrid functional.

The nature of vertical excited states of dyes containing metals for DSSC applications: insights from TD-DFT and density based indexes

Transition metal complexes, typically Ru-based complexes, are the most efficient dyes used in dye-sensitized solar cells. The absorption spectra of these molecules generally involve numerous electronic transitions, which are not equivalent for the conversion of the light into electricity. In the present manuscript, an analysis of each electronic transition of selected inorganic complexes is performed based on the variation of the electronic density upon light absorption. To this end, a series of indices recently proposed in the literature is applied. The main conclusions of this work are twofold: from a methodological point of view, global hybrid functionals confirm their robustness for studying the electronic transitions of these compounds and from an application oriented point of view it is clear that the most intense transitions are not necessarily the most efficient ones for the light conversion.