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We propose two analytical expressions aiming to rationalize the spin-component-scaled (SCS) and spin-opposite-scaled (SOS) schemes for double-hybrid exchange-correlation density-functionals. Their performances are extensively tested within the framework of the nonempirical quadratic integrand double-hybrid (QIDH) model on energetic properties included into the very large GMTKN30 benchmark database, and on structural properties of semirigid medium-sized organic compounds. The SOS variant is revealed as a less computationally demanding alternative to reach the accuracy of the original QIDH model without losing any theoretical background. [ABSTRACT FROM AUTHOR]

A theoretical study was conducted to analyze bonding in lanthanide trihalides using density functional techniques. The local spin density approximation accommodated exchange and correlation potentials. Geometries were then optimized in the pyramidal and planar conformations by an energy-only minimizer. Results indicated the effectiveness of density functional computations to duplicate both structural and thermodynamic experimental information on the compounds.

A-quantum-mechanical technique determines the structural conformations and electron spin resonance characteristics of glycine radicals. The planar or quasiplanar conformations having delocalized pie electrons and minimum energies, are the most preferred geometries. The glycine radicals have slightly pyramidal NH2 units and slightly bent radical centers. The H-alpha and NH hydrogens are trans to each other along the mean molecular plane.

A general quantum mechanical protocol applicable for flexible open-shell system helps examine the conformational behavior and magnetic characteristics of organic radicals derived from glycine, which are associated in peptide chains. Effective pi-electron delocalization accounts for the energetical accessibility of planar or quasi-planar conformations. Theoretical predictions of hyperfine splittings for these conformations agree with the experimental results. Intrinsic conformational preferences in this residue are not altered by long-range interactions.

The combination of a Monte Carlo (MC) sampling of the configurational space with time dependent‐density functional theory (TD‐DFT) to estimate vertical excitations energies has been applied to compute the absorption spectra of a family of merocyanine dyes in both their monomeric and dimeric forms. These results have been compared to those obtained using a static DFT/TD‐DFT approach as well as to the available experimental spectra. Though suffering of the limitations related to the use of DFT and TD‐DFT for this type of systems, our data clearly show that the classical MC sampling provides a suitable alternative to classical molecular dynamics to explore the structural flexibility of these donor‐acceptor (D‐π‐A) chromophores enabling a realistic description of the potential energy surface of both their monomers and aggregates (here dimers) and thus of their spectra. Overall, the combination of MC sampling with quantum mechanics (TD‐DFT) calculations, carried out in implicit dioxane solvent on random snapshots, provides a workable compromise to solve the combined challenge of accuracy and time‐consuming problem not only for merocyanines momers, but also for their dimers, up to now less investigated. Indeed, the simulated absorption spectra fairly agree with the experimental ones, suggesting the general reliability of the method. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A comprehensive theoretical investigation of the MLaNb2O7 (M = H, Li, Na, K, Rb, and Cs) series of ion-exchangeable layered perovskite is presented. These perovskites are in particular interesting in view of their potential applications as inorganic supports for the design of new hybrid inorganic-organic proton conductors. In particular, their structural and electronic properties have been investigated by periodic calculations in the framework of Density Functional Theory, using different exchange-correlation functionals. A general very good agreement with the available experimental (XRD, NPD, and EXAFS) data has been found. The structure of the protonated HLaNb2O7 form has also been further clarified and a new tetragonal space group is proposed for this compound, better reproducing the experimental cell parameters and yielding to a more realistic picture of the system. The electronic investigation highlighted that all the compounds considered are very similar to each other and that the interaction between interlayer cations and perovskite slabs is purely ionic, except for the proton that is, instead, covalently bound. [ABSTRACT FROM AUTHOR]

We extend our previous definition of the metric Δr for electronic excitations in the framework of the time-dependent density functional theory [C. A. Guido, P. Cortona, B. Mennucci, and C. Adamo, J. Chem. Theory Comput.9, 3118 (2013)], by including a measure of the difference of electronic position variances in passing from occupied to virtual orbitals. This new definition, called Γ, permits applications in those situations where the Δr-index is not helpful: transitions in centrosymmetric systems and Rydberg excitations. The Γ-metric is then extended by using the Natural Transition Orbitals, thus providing an intuitive picture of how locally the electron density changes during the electronic transitions. Furthermore, the Γ values give insight about the functional performances in reproducing different type of transitions, and allow one to define a "confidence radius" for GGA and hybrid functionals. [ABSTRACT FROM AUTHOR]

The perylene bisimide derivative Paliogen Black (P‐black) is proposed as a new chromogenic probe that shows visible (vis) and near‐infrared (NIR) responses after mechanical solicitations of host linear low‐density polyethylene (LLDPE) films. P‐black is reported to display strong absorption in the vis spectrum and unusual reflective and cooling features in the NIR region. Uniaxial deformation of the 2.5, 5, and 10 wt% P‐black/LLDPE films yields a dichroic absorption under polarized light with color variations attributed by the computational analysis to the distinct anisotropic behavior of the transition dipole moments of P‐black chromophores. When LLDPE films are deformed, P‐black aggregates reduce their size from ≈30–40 µm to ≈5–10 µm that, in turn, causes reflectivity losses of about 30–40% at the maximum elongation. This gives rise to warming of 5–6 °C of the locally oriented film placed in contact with a black substrate under the illumination with an IR lamp for 5 s. These features combined with the high sensitivity of the vis‐NIR response toward mechanical solicitations render P‐black as a new solution to detect uniaxial deformations of plastic films through both optical and thermal outputs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this work, we present the application of the hybrid short-range density functional theory/long-range MP2 energy gradients to the bond length alternation in polymethineimine and polyacetylene conjugated oligomers. Compared to other density functional calculations, our results are quite superior, even to fourth rung functionals, usually better than MP2 and very close to the available CCSD(T) values. [ABSTRACT FROM AUTHOR]

A first (local) bridge between Kohn–Sham density functional theory and the quantum theory of atoms in molecules of Bader is built by means of a second order reduced density gradient expansion of the exchange-correlation energy density at a given bond critical point. This approach leads to the definition of new “mixed” descriptors that are particularly useful for the classification of the chemical interactions for which the traditional atoms in molecules characterization reveals insufficient, as for instance the distinction between hydrogen and agostic bonds. [ABSTRACT FROM AUTHOR]

A comprehensive investigation of one of the basic components of ZnO-based dye-sensitized solar cells (DSSC) is presented, carried out using hybrid density functionals combined to a periodic formalism. Both semiconductor bulk and surfaces are discussed thoroughly, with a particular attention to structural and electronic aspects. Next, three possible adsorption modes of formic acid are compared and discussed at the same level of theory. The results confirm that formic acid appears as a suitable choice for an efficient anchoring of large organic molecules, such as the dyes commonly used for DSSC, to semiconductor surfaces since it allows both a stable adsorption and few but significant contributions to the density of states for all adsorption modes considered. More in general, our results suggest that hybrid functionals and, in particular the parameter free PBE0 (PBE denotes Perdew–Burke–Ernzerhof), can be considered as a reliable tool for modeling complex molecule-semiconductors interfaces such as the one of interest in DSSC, thus providing a powerful computational protocol for the in silico design of new systems for photovoltaic applications. [ABSTRACT FROM AUTHOR]

A new parameter-free correlation functional based on the local Ragot-Cortona approach [J. Chem. Phys. 121, 7671 (2004)] is presented. This functional rests on a single ansatz for the gradient correction enhancement factor: it is assumed to be given by a simple analytic expression satisfying some exact conditions and containing two coefficients. These coefficients are determined without implementing the functional and without using a fitting procedure to experimental data. Their values are determined by requiring that the functional gives a correct average reduced density gradient for atoms, which, to some extent, can be considered an intrinsic atomic property. The correlation functional is then coupled with the Perdew-Burke-Erzernhof (PBE) exchange and compared with the original PBE approach as well as with some other pure density or hybrid approaches. Standard tests for atomic and molecular systems show that our new functional significantly improves on PBE, showing very interesting properties. [ABSTRACT FROM AUTHOR]

The first n→π* transitions of 18 nitroso and 16 thiocarbonyl dyes have been computed by time-dependent density functional theory (TD-DFT) using pure as well as global and range-separated hybrid functionals. It turns out that the accuracy of all hybrids is relatively similar, i.e., the inclusion of a growing fraction of exact exchange does neither worsen nor improve significantly the raw TD-DFT estimations. However, after a simple linear regression, it appears that the range-separated hybrids provide a better accuracy than global hybrids. [ABSTRACT FROM AUTHOR]