Evaluating Noncovalent Interactions in Halogenated Molecules with Double-Hybrid Functionals and a Dedicated Small Basis Set.

We present here an extension of our recently developed PBE-QIDH/DH-SVPD basis set to halogen atoms, with the aim of obtaining, for weakly interacting halogenated molecules, interaction energies close to those provided by a large basis set (def2-TZVPP) coupled to empirical dispersion potential. The core of our approach is the split-valence basis set, DH-SVPD, that has been developed for F, Cl, Br, and I atoms using a self-consistent formula, containing only energy terms computed for dimers and the corresponding monomers at the same level of theory. The basis set developed considering four systems, one for each halogen atoms, has been then tested on the X40, X4 × 10 benchmarks as well as on other two, less standard, data sets. Finally, a large system (380 atoms) has been also considered as a "crash" test. Our results show that the simple and nonempirical PBE-QIDH/DH-SVPD approach is able to provide accurate results for interaction energies of all the considered systems and can thus be considered as a cheaper alternative to DH functionals paired with empirical dispersion corrections and a large basis set of triple-ζ quality.

A combined Monte Carlo/DFT approach to simulate UV-vis spectra of molecules and aggregates: Merocyanine dyes as a case study.

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.

Beyond Chemical Accuracy for Alkane Thermochemistry: The DHthermo Approach.

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.