Bandgaps of atomically precise graphene nanoribbons and Occam's razor.

Rationalization of the "bulk" (Δ) or "zigzag-end" (Δ) energy gaps of atomically precise armchair graphene nanoribbons (AGNRs), which are directly related to fundamental applications in nanoelectronics, could be challenging and largely controversial with respect to their magnitude, origin, substrate influence (Δ), and spin-polarization, among others. Hereby a simple self-consistent and "economical" interpretation is presented, in full accordance with Occam's simplicity principle, which is highly successful (within less than 1%) in predicting all energy gaps of the 5-, 7-, and 9-AGNRs, in contrast to other complicated and/or contradicting prevailing views in the literature for Δ, Δ, and Δ. The present approach is based on "appropriate" DFT (TDDFT) calculations, general symmetry principles, and plausibility arguments.

4 + 2 = 6? A Geometrical Approach to Aromaticity?

Using a simple but powerful geometrical/topological notion of aromaticity based on the shell model (Zdetsis, A. D. , , 17526-17536) and the bipartite topology, we uncover, on top of the geometrical virtual "equivalence" of the fundamental Hückel and Clar rules of aromaticity, the significance of empty peripheral rings, which are shown to be linked to zigzag edge states.

Bridging the Physics and Chemistry of Graphene(s): From Hückel's Aromaticity to Dirac's Cones and Topological Insulators.

By bridging graphene and benzene through a well-defined sequence of polycyclic aromatic hydrocarbons and their inherent shell structure, it is shown that graphene is actually a coherent arrangement of interwoven benzene molecules, coordinated by aromaticity, shell structure, and topology, all interrelated and microscopically realized through dynamical flipping of the atomic p-orbitals, playing the role of pseudospins or "qubits". This renders graphene resonance structure, "resonating" between two complementary aromaticity patterns, involving 2, → ∞ Kekulé type of resonances, resulting in "robust electronic coherence", with a dual "molecular crystalline" nature, and two valence-conduction bands of opposite parity, driven by inversion symmetry competition, which is essentially a "molecule-versus-crystal" competition, in accordance with topological insulators and many-body theory. The "average picture" converges to the usual band structure with two aromatic π-electrons per ring, and with the fingerprints of inversion competition at the -symmetric Dirac points, which for rectangular nanographene(s) appear as gapless topological edge states without real spin polarization, in contrast to opposite claims.

Interrelation of Aromaticity and Conductivity of Graphene Dots/Antidots and Related Nanostructures.

It is illustrated and computationally verified by ab initio density functional theory and simple but powerful order-of-magnitude arguments, based on deformation energy Δ in relation to the uncertainty principle, that the conductivity and aromaticity of graphene and graphene-based structures, such as graphene dots, antidots, and nanoribbons, are negatively interrelated for π aromatic structures, in agreement with recent experimental data. However, for σ aromaticity, the interrelation could be positive, especially for extended periodic structures. We predict that the conductivity of rectangular graphene dots and antidots, is anisotropic with much larger magnitude along the direction perpendicular to the zigzag edges, compared to the conductivity in direction parallel to them.

Ab initio theoretical investigation of beryllium and beryllium hydride nanoparticles and nanocrystals with implications for the corresponding infinite systems.

With the initial motivation of optimizing hydrogen storage in beryllium nanocrystals, we have thoroughly and systematically studied the structural, cohesive, and electronic properties of Ben and BenHxn (n = 2-160, x = 0.1-2.4) nanoparticles as a function of both size (n) and hydrogen content (x), using density functional theory with a properly selected meta-hybrid functional and high level coupled cluster CCSD(T) theory for comparison.

Ab initio study of magnesium and magnesium hydride nanoclusters and nanocrystals: examining optimal structures and compositions for efficient hydrogen storage.

On the basis of the attractive possibility of efficient hydrogen storage in light metal hydrides, we have examined a large variety of Mg(n)H(m) nanoclusters and (MgH(2))(n) nanocrystals (n = 2-216, m = 2-436) using high level coupled cluster, CCSD(T), ab initio methods, and judicially chosen density functional calculations of comparable quality and (near chemical) accuracy. Our calculated desorption energies as a function of size and percentage of hydrogen have pinpointed optimal regions of sizes and concentrations of hydrogen which are in full agreement with recent experimental findings. Furthermore, our results reproduce the experimental desorption energy of 75.

Structural and static electric response properties of highly symmetric lithiated silicon cages: theoretical predictions.

It is shown by density functional theory calculations that high symmetry silicon cages can be designed by coating with Li atoms. The resulting highly symmetric lithiated silicon cages (up to D(5d) symmetry) are low-lying true minima of the energy hypersurface with binding energies of the order of 4.6 eV per Si atom and moderate highest occupied molecular orbital-lowest unoccupied molecular orbital gaps.

Designing novel Sn-Bi, Si-C and Ge-C nanostructures, using simple theoretical chemical similarities.

A framework of simple, transparent and powerful concepts is presented which is based on isoelectronic (or isovalent) principles, analogies, regularities and similarities. These analogies could be considered as conceptual extensions of the periodical table of the elements, assuming that two atoms or molecules having the same number of valence electrons would be expected to have similar or homologous properties. In addition, such similar moieties should be able, in principle, to replace each other in more complex structures and nanocomposites.

Novel pentagonal silicon rings and nanowheels stabilized by flat pentacoordinate carbon(s).

It is predicted by accurate density functional and coupled-cluster theory that planar [Si(5)C](2-) and [Si(5)C](1-) rings can be stabilized by flat pentacoordinate carbon-silicon bonds. The energy difference of the [Si(5)C](2-) dianion from the lowest energy three-dimensional isomer is about 12.2 kcal∕mol at the level of the density functional theory using the Becke 3-parameter (exchange), Lee, Yang and Parr functional, and the triple-ζ doubly polarized basis sets.

Rationalizing and functionalizing stannaspherene: Very stable stannaspherene "alloys".

It is illustrated here by ab initio calculations based on density functional theory and other high level methods that the high stability of the icosahedral Sn(12) (2-) dianion known as stannaspherene, reflects stability toward ionization rather than cohesion. This could be also connected with novel fluxional rearrangements and paths of Sn(12) (1-) leading eventually to Sn(12) (2-) involving charge transfer. In view of the very similar structural and electronic properties with the corresponding isovalent borane (B(12)H(12))(2-), it is demonstrated that stannaspherene can be further rationalized and functionalized on the basis of an isolobal analogy between group 14 clusters and isovalent boranes, carboranes, and bisboranes.

Silicon-bismuth and germanium-bismuth clusters of high stability.

Mixed metal-semiconductor clusters of the form Bi2Si(n-2) and Bi2Ge(n-2), n = 3-8, 12, are studied theoretically by ab initio methods including density functional theory with the hybrid B3LYP functional, second-order perturbation, and coupled cluster CCSD(T) theory using the doubly polarized TZV2P basis sets. These clusters are characterized by high stability and symmetry and relatively large highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) energy gaps. It is shown that the lower energy structures of these clusters and their bonding and electronic characteristics are fully compatible with very powerful stability rules and structural laws similar to the ones for the corresponding isovalent boranes, carboranes, and bisboranes.

Success and pitfalls of the Si(n-2)C(2)H(2)-C(2)B(n-2)H(n) isolobal analogy: Depth and breadth of the boron connection.

The extent and depth of the so-called boron connection suggested recently by the present author [J. Chem. Phys.

The boron connection: a parallel description of aromatic, bonding, and structural characteristics of hydrogenated silicon-carbon clusters and isovalent carboranes.

The aromatic, bonding, and structural characteristics of the Si 4C 2H 2-C 2B 4H 6, Si 2C 4H 4-C 4B 2H 6, and other Si n C 2H 2-C 2B n H n+2 ( n = 1, 2, 3, 5) isovalent pairs are studied using density functional theory (DFT) and coupled cluster methods to fully illustrate the homology of the two species. This homology, which is based on the replacement of the carborane B-H units by isovalent Si atoms, is extended to all three characteristics (structural, electronic, and aromatic) and includes all three lowest-energy structures of the isovalent pairs. This type of "boron connection", which has been tested for silicon clusters recently, seems to be a valid and extremely useful concept.

A new class of silicon-carbon clusters: a full study of the hydrogenated SinC2H2, n=3,4,5, clusters in comparison with their isoelectronic carboranes C2BnHn+2.

The structural and electronic characteristics of the Si(n)C(2)H(2), n=3,4,5, clusters are studied by ab initio calculations based on coupled cluster and density functional theory using the hybrid B3LYP functional. It is demonstrated that all three clusters are structurally and electronically homologous to the corresponding isoelectronic organometallic carboranes C(2)B(n)H(n+2). This homology, which is in full agreement with the analogy of Si(6) (2-) and B(6)H(6) (2-) demonstrated recently by the author [J.

High-stability hydrogenated silicon-carbon clusters: a full study of Si2C2H2 in comparison to Si2C2, C2B2H4, and other similar species.

The structural and electronic characteristics of the Si2C2H2 and Si2C2 clusters are studied by ab initio calculations based on coupled cluster and density functional theory using the hybrid B3LYP functional. In addition, similar species, such as SiC2H2 and Si3C2H2, are also studied for comparison. It is illustrated that the lowest energy structures of all three hydrogenated clusters, which have the general form Si(n)(CH)2, n = 1, 2, 3, are fully analogous to the structures of the corresponding organometallic isovalent carboranes.

Analogy of silicon clusters with deltahedral boranes: how far can it go? Reexamining the structure of Sin and Sin 2-, n=5-13 clusters.

Silicon clusters of 5 up to 13 atoms, Si(n), n=5-13, and their dianions are studied in the light of an anticipated analogy with the corresponding isoelectronic boranes suggested recently by Zdetsis [J. Chem. Phys.

Stabilization of flat aromatic Si6 rings analogous to benzene: ab initio theoretical prediction.

It is shown by ab initio calculations, based on density functional (DFT/B3LYP), and high level coupled-cluster [CCSD(T)] and quadratic CI [QCISD(T)] methods, that flat aromatic silicon structures analogous to benzene (C6H6) can be stabilized in the presence of lithium. The resulting planar Si6Li6 structure is both stable and aromatic, sharing many key characteristics with benzene. To facilitate possible synthesis and characterization of these species, routes of formation with high exothermicity are suggested and several spectral properties (including optical absorption, infrared, and Raman) are calculated.

Fluxional and aromatic behavior in small magic silicon clusters: a full ab initio study of Si(n), Si(n) (1-), Si(n) (2-), and Si(n) (1+), n=6, 10 clusters.

The structural, electronic, vibrational, optical, magnetic, and aromatic characteristics of Si(n), Si(n) (1-), Si(n) (2-), and Si(n) (1+), clusters have been calculated very accurately with a variety of high level ab initio techniques. These calculations have been performed with the aim to clarify existing ambiguities in the literature and to bring up the fluxional and aromatic characteristics of these species. The fluxional behavior, according to earlier conjecture of the present author, could be connected to the magic property.