Juan-Carlos Sancho-Garcia

Assessment of double-hybrid energy functionals for π-conjugated systems

There have been tremendous efforts in the past decade on the use of computational methods for conjugated systems. Their properties and energetics are often described by density functional theory calculations which, however, are known to face a challenge when dealing with these systems since serious and systematic errors with popular density functionals occur, specially in the case of having stacked or sterically overcrowded aromatic systems, and discourage their use as a black box technique. We overcome here this shortcoming by applying recently developed dispersion-corrected double-hybrid density functionals (B2PLYP) in search of greater yet wide accuracy with little more computational effort. Interestingly, we have derived a related method (B2piPLYP), which has been thoroughly assessed against a set of databases and reactions of the most interest, and works better for this subclass of systems. The deviations with respect to benchmark or experimental values are found to be in the reasonably low range of 1-2 kcal/mol when a correction for the dispersion interactions is added and, most importantly, without suffering the large and systematic errors that are common in former yet conventional methods.

Stabilizing and Modulating Color by Copigmentation: Insights from Theory and Experiment

Natural anthocyanin pigments/dyes and phenolic copigments/co-dyes form noncovalent complexes, which stabilize and modulate (in particular blue, violet, and red) colors in flowers, berries, and food products derived from them (including wines, jams, purees, and syrups). This noncovalent association and their electronic and optical implications constitute the copigmentation phenomenon. Over the past decade, experimental and theoretical studies have enabled a molecular understanding of copigmentation. This review revisits this phenomenon to provide a comprehensive description of the nature of binding (the dispersion and electrostatic components of π-π stacking, the hydrophobic effect, and possible hydrogen-bonding between pigment and copigment) and of spectral modifications occurring in copigmentation complexes, in which charge transfer plays an important role. Particular attention is paid to applications of copigmentation in food chemistry.

Accurate calculation of transport properties for organic molecular semiconductors with spin-component scaled MP2 and modern density functional theory methods

At ambient temperatures, intermolecular hopping of charge carriers dominates the field effect mobility and thus the performance of organic molecular semiconductors for organic-based electronic devices. We have used a wide variety of modern and accurate computational methods to calculate the main parameters associated with charge transport, taking oligoacenes, and its derivatives as the exemplary organic materials. We tackle the problem from a combined inter- and intramolecular approach, in which the parameters are calculated for an isolated single molecule concomitantly with the stability of the dimers found in experimentally determined crystalline structures. Considering that most of the future applications within the field would need a full understanding of the transport mechanism, we assess the reliability of the methods to be employed according to the nature of the problem. Finally, we perform a computationally guided molecular engineering of a new set of materials derived from tetracene (rubrene and highly twisted oligoacenes) which allows to robustly anticipate the reasons for their expected performance in organic-based electronic devices.

Assessing a new nonempirical density functional: Difficulties in treating π-conjugation effects

The reliability of the Tao-Perdew-Staroverov-Scuseria (TPSS) exchange-correlation functional for the description of conjugation effects in model pi-conjugated systems has been thoroughly assessed through the calculation of torsion energy profiles. The functional reproduces qualitatively the shape of torsional potentials but, interestingly, the mixing of TPSS and exact exchange governs the quantitative results: thus, well-defined hybrid extensions of the functional are consistently employed to improve the results. The hybrid approaches led to more accurate descriptions of conjugation effects but, however, the finest performance along the whole range of dihedral angles was obtained by a customized mixing of pure or hybrid TPSS functionals and wave function methods in a multicoefficient fashion. Despite the successful construction of this nonempirical functional, higher rungs of the ladder of methods in which TPSS is based are hoped to reduce the errors with respect to reference data for pi-conjugated systems.

Improved accuracy with medium cost computational methods for the evaluation of bond length alternation of increasingly long oligoacetylenes

The difference between the length of the central carbon-carbon bond and that of the adjacent flanked double bonds in polymers such as polyacetylene is closely related to their electronic properties and plays a central role in their conductivity upon doping. Simple as it seems, this bond length alternation (BLA) is a difficult test for many theoretical methods. Accurate coupled-cluster (CC) benchmark values are difficult to obtain even for small- and medium-sized oligoacetylenes due to their intrinsic computational limitations. Here we present a computationally much cheaper alternative to obtain accurate benchmark BLA values, even for large polyacetylene oligomers, by using the so-called spin-component scaled Møller-Plesset perturbation theory up to second order (SCS-MP2). Comparison between these new benchmark BLA with those provided by density functional theory (DFT) calculations shows a large dispersion of the results depending on the amount of exact exchange used in the exchange-correlation functional. We find that the percentage of exact exchange needed to accurately reproduce the new benchmark BLA is much larger than previously thought when comparison was made with values obtained using the MP2 method.

A comparison between DFT and other ab initio schemes on the activation energy in the automerization of cyclobutadiene

Abstract The influence of the multireference character in the transition state for the automerization reaction of cyclobutadiene is considered. We have analyzed two forms of taking into account this effect: either by the use of two-body density functionals or traditional density functional theory (DFT) correlation functionals conveniently modified. Comparison has also been made with conventional density functional theory Kohn–Sham (DFT–KS) [15] approaches. It is shown that only when the aforementioned multideterminantal character is included in the computational scheme is the activation energy in accord with accurate benchmark calculations.

Application of recent double-hybrid density functionals to low-lying singlet-singlet excitation energies of large organic compounds

The present work assesses some recently developed double-hybrid density functionals (B2π-PLYP, PBE0-DH, and PBE0-2) using linear-response Tamm-Dancoff Time-Dependent Density Functional Theory. This assessment is achieved against experimentally derived low-lying excitation energies of large organic dyes of recent interest, including some excitations dominated by charge-transfer transitions. Comparisons are made with some of the best-performing methods established from the literature, such as PBE0 or B3LYP hybrid or the recently proposed B2-PLYP and B2GP-PLYP double-hybrid models, to ascertain their quality and robustness on equal footing. The accuracy of parameter-free or empirical forms of double-hybrid functionals is also briefly discussed. Generally speaking, it turns out that double-hybrid expressions always provide more accurate estimates than corresponding hybrid methods. Double-hybrid functionals actually reach averaged accuracies of 0.2 eV, that can be admittedly considered close to any intended accuracy limit within the present theoretical framework.

Obtaining the lattice energy of the anthracene crystal by modern yet affordable first-principles methods

The non-covalent interactions in organic molecules are known to drive their self-assembly to form molecular crystals. We compare, in the case of anthracene and against experimental (electronic-only) sublimation energy, how modern quantum-chemical methods are able to calculate this cohesive energy taking into account all the interactions between occurring dimers in both first-and second-shells. These include both O(N(6))- and O(N(5))-scaling methods, Local Pair Natural Orbital-parameterized Coupled-Cluster Single and Double, and Spin-Component-Scaled-Møller-Plesset perturbation theory at second-order, respectively, as well as the most modern family of conceived density functionals: double-hybrid expressions in several variants (B2-PLYP, mPW2-PLYP, PWPB95) with customized dispersion corrections (-D3 and -NL). All-in-all, it is shown that these methods behave very accurately producing errors in the 1-2 kJ/mol range with respect to the experimental value taken into account the experimental uncertainty. These methods are thus confirmed as excellent tools for studying all kinds of interactions in chemical systems.

Usefulness of the Colle–Salvetti model for the treatment of the nondynamic correlation

In this work, the behavior of the Colle–Salvetti correlation functional is examined for strongly correlated systems with non-negligible nondynamic effects. Used with an appropriate multideterminantal wave function, it is able to reproduce accurately previous multireference coupled-cluster results for the problem of the automerization of cyclobutadiene, as well as to provide the correct energetical profiles for diatomic molecules under dissociation. The results confirm the current quality of the functional for complicated chemical problems, in spite of the fact that the functional does not satisfy some known exact properties.

Nitrobenzene rotational energy barrier: A survey of several ab initio methods

A detailed study on the torsional potential of nitrobenzene is performed by using state-of-the-art ab initio methods, including density functional theory (DFT) ones. Special emphasis is given to basis set incompleteness in order to get complete agreement with experimental results. On the other hand, the DFT derived the energy difference between planar and perpendicular conformers is mostly dominated by the proportion of the HF exchange contribution to the exchange-correlation functional. A systematic application of hybrid schemes, energy decomposition analysis, and scan of the conjugative, steric, and weak interactions effects along the torsion will be used to rationalize the torsional profiles.