Long-Range Effects in Topologically Defective Arm-Chair Graphene Nanoribbons.

The electronic structure of 7/9-AGNR superlattices with up to eight unit cells has been studied by means of state-of-the-art Density Functional Theory (DFT) and also by two model Hamiltonians, the first one including only local interactions (Hubbard model, Hu) while the second one is extended to allow long-range Coulomb interactions (Pariser, Parr and Pople model, PPP). Both are solved within mean field approximation. At this approximation level, our calculations show that 7/9 interfaces are better described by spin non-polarized solutions than by spin-polarized wavefunctions.

Electronic structure of rhombus-shaped nanographenes: system size evolution from closed- to open-shell ground states.

We theoretically study and characterize a set of rhombus-shaped nanographenes of increasing size, or -rhombenes, where = 2-6, displaying zigzag edges leading to an enhancement of the (poly)radicaloid nature and the appearance of intrinsic magnetism as a function of . Due to that system-dependent radicaloid nature, we employ spin-flip methods able to capture the challenging physics of the problem, thus providing accurate energy differences between high- and low-spin solutions. The theoretical predictions agree with the experimentally available magnetic exchange coupling for the recently synthesized 5-rhombene, as well as with the size at which the transition from a closed-shell to an open-shell ground-state solution occurs.

-Acenoacene molecules: tuning of the singlet and triplet excitation energies by modifying their radical character.

The energy difference between singlet and triplet excitons, or Δ, is a key parameter for novel light-emission mechanisms (, TADF or thermally activated delayed fluorescence) or other photoactivated processes. We have studied a set of conjugated molecules (-acenoacenes and their heteroatom-doped analogues) to observe the evolution of their excited-state properties upon increasing the system size with and without substitution with a pair of N atoms. Since these molecules exhibit a (ground-state) diradicaloid character, together with marked correlation effects influencing the excited-states formed, we have applied a variety of theoretical methods (FT-DFT, TD-DFT, SF-TD-DFT, CIS, CIS(D), SCS-CC2, SA-CASSCF, and SC-NEVPT2) to bracket the accuracy of the results while concomitantly providing insights into electronic structure.

Probability distribution for heat exchange in plastic deformation.

Fluctuation theorems allow one to obtain equilibrium information from nonequilibrium experiments. The probability distribution function of the relevant magnitude measured along the irreversible nonequilibrium trajectories is an essential ingredient of fluctuation theorems. In small systems, where fluctuations can be larger than average values, probability distribution functions often deviate from being Gaussian, showing long tails, mostly exponential, and usually strongly asymmetric.

Singlet-Triplet Excited-State Inversion in Heptazine and Related Molecules: Assessment of TD-DFT and ab initio Methods.

We have investigated the origin of the S -T energy levels inversion for heptazine, and other N-doped π-conjugated hydrocarbons, leading thus to an unusually negative singlet-triplet energy gap ( ). Since this inversion might rely on substantial doubly-excited configurations to the S and/or T wavefunctions, we have systematically applied multi-configurational SA-CASSCF and SC-NEVPT2 methods, SCS-corrected CC2 and ADC(2) approaches, and linear-response TD-DFT, to analyze if the latter method could also face this challenging issue. We have also extended the study to B-doped π-conjugated systems, to see the effect of chemical composition on the results.

Investigating the (Poly)Radicaloid Nature of Real-World Organic Compounds with DFT-Based Methods.

Recent advances in the synthesis of stable organic (open-shell) polyradicaloids have opened their application as active compounds for emerging technologies. These systems typically exhibit small energy differences between states with different spin multiplicities, which are intrinsically difficult to calculate by theoretical methods. We thus apply here some DFT-based variants (FT-DFT, SF-DFT, and SF-TDDFT) on a test set of large and real-world molecules, as test systems for which such energy differences are experimentally available, also comparing systematically with RAS-SF results to infer if shortcomings of previous DFT applications are corrected.

Transport and Optical Gaps in Amorphous Organic Molecular Materials.

The standard procedure to identify the hole- or electron-acceptor character of amorphous organic materials used in OLEDs is to look at the values of a pair of basic parameters, namely, the ionization potential (IP) and the electron affinity (EA). Recently, using published experimental data, the present authors showed that only IP matters, i.e.

The role of topology in organic molecules: origin and comparison of the radical character in linear and cyclic oligoacenes and related oligomers.

We discuss the nature of electron-correlation effects in carbon nanorings and nanobelts using an analysis tool known as fractional occupation number weighted electron density (ρ) and the RAS-SF method, revealing for the first time significant differences in static correlation effects depending on how the rings (i.e. chemical units) are fused and/or connected until closing the loop.

Herringbone Pattern and CH-π Bonding in the Crystal Architecture of Linear Polycyclic Aromatic Hydrocarbons.

The herringbone pattern is a pervasive structural motive found in most molecular crystals involving aromatic compounds. A plot of the experimental sublimation enthalpies of members of increasing size of the acene, phenacene and p-phenyl families versus the number of carbons uncovers a linear relationship between the two magnitudes, suggesting a major role of CH-π bonding. In this work we undertake the task of evaluating the relevance of the edge-to-face interaction (or CH-π bond) in the overall reticular energy of the crystal, to quantitatively assess the importance of this structural element.

Can model Hamiltonians describe the electron-electron interaction in π-conjugated systems? PAH and graphene.

Model Hamiltonians have been, and still are, a valuable tool for investigating the electronic structure of systems for which mean field theories work poorly. This review will concentrate on the application of Pariser-Parr-Pople (PPP) and Hubbard Hamiltonians to investigate some relevant properties of polycyclic aromatic hydrocarbons (PAH) and graphene. When presenting these two Hamiltonians we will resort to second quantisation which, although not the way chosen in its original proposal of the former, is much clearer.

Intra- and intermolecular dispersion interactions in [N]cycloparaphenylenes: do they influence their structural and electronic properties?

Cycloparaphenylenes (CPPs) are nanosized structures with unique isolated and bulk properties, and are synthetic targets for the template-driven bottom-up synthesis of carbon nanotubes. Thus, a systematic understanding of the supramolecular order at the nanoscale is of utmost relevance for molecular engineering. In this study, it is found that intramolecular noncovalent (dispersion) interactions must be taken into account for obtaining accurate estimates of the structural and optoelectronic properties of [n]CPP compounds, and their influence as the number of repeat units increases from n=4 to n=12 is also analyzed, both in the gas phase and in solution.

Cyclobutadiene automerization and rotation of ethylene: energetics of the barriers by using spin-polarized wave functions.

Spin-projected spin polarized Møller-Plesset and spin polarized coupled clusters calculations have been made to estimate the cyclobutadiene automerization, the ethylene torsion barriers in their ground state, and the gap between the singlet and triplet states of ethylene. The results have been obtained optimizing the geometries at MP4 and/or CCSD levels, by an extensive Gaussian basis set. A comparative analysis with more complex calculations, up to MP5 and CCSDTQP, together with others from the literature, have also been made, showing the efficacy of using spin-polarized wave functions as a reference wave function for Møller-Plesset and coupled clusters calculations, in such problems.

Non-Gaussian tails in the probability distribution function of heat exchanged during isothermal stretching of aluminum and gold nanowires.

The heat exchanged upon isothermal (0.5-200 K) stretching of aluminum and gold nanowires has been calculated by means of molecular dynamics. Atoms at fixed positions with velocities randomly distributed according to Maxwell distribution were taken as initial conditions.

First-principles phase-coherent transport in metallic nanotubes with realistic contacts.

We present first-principles calculations of phase coherent electron transport in a carbon nanotube (CNT) with realistic contacts. We focus on the zero-bias response of open metallic CNT's considering two archetypal contact geometries (end and side) and three commonly used metals as electrodes (Al, Au, and Ti). Our ab initio electrical transport calculations make, for the first time, quantitative predictions on the contact transparency and the transport properties of finite metallic CNT's.

Prediction of the secondary structure and functional sites of major histocompatibility complex molecules.

A method for the theoretical prediction of the antigenic determinants and the antigen-interactive receptor sites of immunological proteins from their primary structure would constitute a useful tool for their study. Such a method developed in this laboratory uses hydrophilicity, accessibility, flexibility, and recognition profiles, together with the predicted secondary structure (alpha-helices, beta-sheets, and turns). The secondary structure is determined by a modification of the method of Lim (1974), as described below.