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.

The so-called protobranching phenomenon, that is the greater stability of branched alkanes with respect to their linear isomers, represents an interesting challenge for approaches based on density functional theory (DFT), since it requires a balanced description of several electronic effects, including (intramolecular) dispersion forces. Here, we investigate this problem using a protocol recently developed based on double-hybrid functionals and a small basis set, DH-SVPD, suited for noncovalent interactions. The energies of bond separation reactions (BSR), defined on the basis of an isodesmic principle, are taken as reference properties for the evaluation of 15 DFT approaches. The obtained results show that error lower than the so-called “chemical accuracy” (<1.0 kcal/mol) can be obtained by the proposed protocol on both relative reaction energies and enthalpies. These results are then verified on the standard BSR36 data set and support the proposition of our computational protocol, named DHthermo, where any DH functional, such as PBE-QIDH or B2PLYP, provides accurate results when coupled to an empirical dispersion correction and the DH-SVPD basis set. This protocol not only gives subchemical accuracy on the thermochemistry of alkanes but it is extremely easy to use with common quantum-chemistry codes.

DH-SVPD is a tailored atomic basis set originally developed to enhance the domain of applicability of double-hybrid density functionals to large molecular systems in weak interactions. In combination with any density functional belonging to this approximation, it provides an accurate estimate of noncovalent interaction energies at the cost of a double-ζ basis set, without adding a posteriori an empirical dispersion correction. Here, we show that the accuracy/cost ratio observed previously for energy properties can be safely extended to the modeling of structural parameters of small-and medium-sized organic molecules. In particular, we demonstrate that, in combination with the nonempirical PBE-QIDH double hybrid, DH-SVPD is competitive with very large quadruple-ζ basis sets when modeling both covalent and noncovalent structural parameters. K E Y W O R D S covalent and noncovalent structural parameters, density-functional theory, DH-SVPD, double-hybrid approximation, low computational cost

In this study we address the Li-ion de-insertion/insertion mechanisms from/into the lattice of the mixed olivine LiCo1/3Fe1/3Mn1/3PO4 (LCFMP).