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We have developed and implemented on several DEC-5000 work stations, a parallel algorithm suitable for distributed concurrent processing, using the Parallel Virtual Machine (PVM). We have used the “regular crowd” structure, in which crowd... more
We have developed and implemented on several DEC-5000 work stations, a parallel algorithm suitable for distributed concurrent processing, using the Parallel Virtual Machine (PVM). We have used the “regular crowd” structure, in which crowd computations are performed through concurrent processes which are identical. Contrary to the ring structure in which succesive time steps are evolved from one processor to the next, crowd conputations are performed in different groups of space steps. As a basis for the Molecular Dynamics (MD) algorithm, we have taken the MD code of Biswas and Hamann, written in Fortran-77.
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HOMO–LUMO and optical gaps of (CdSe)n nanocrystals appear to have controversial magnitudes and size dependence, which we have rationalized.
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Polyethyleneimine is a very interesting polymer, not only for its extensive use in biological applications, but also for its crystal structure as a double-stranded helix in the anhydrous state. In order to elucidate the electronic bulk... more
Polyethyleneimine is a very interesting polymer, not only for its extensive use in biological applications, but also for its crystal structure as a double-stranded helix in the anhydrous state. In order to elucidate the electronic bulk properties of the crystalline (or linear) polyethyleneimine built from ethylenediamine molecules in anhydrous conditions, we performed ab initio density functional theory calculations on water-free molecular crystal structures. The resulting polymer is a semiconductor with a small band gap: Eg=0.40 eV.
Research Interests: Engineering, Materials Science, Chemical Physics, Medicine, Crystal structure, and 15 morePolymer, Computer Simulation, Density Functional Theory, Physical sciences, Band Gap, Electronic Structure, CHEMICAL SCIENCES, Molecular Conformation, Ab Initio, Local Density Approximation, Molecule, Electric Conductivity, Organic Compound, Polyethyleneimine, and Ethylenediamine
... of distant the origin 1 in set (z1, X-,, x3) 1 FC matrix parameters neighbors 4 4 2 4 1 4..J3 4 (4,4,4) ~( ss pppa ss fi a 2 L.,J~ 12 [5] (0,1,1) 11 o -5 -o ~ ry 3 4,/11 12 [19] ( 4, 4, 4) fi ~ ss 0 0 z VV /1 4 4~J1~ 6 [29] (0.0.2)... more
... of distant the origin 1 in set (z1, X-,, x3) 1 FC matrix parameters neighbors 4 4 2 4 1 4..J3 4 (4,4,4) ~( ss pppa ss fi a 2 L.,J~ 12 [5] (0,1,1) 11 o -5 -o ~ ry 3 4,/11 12 [19] ( 4, 4, 4) fi ~ ss 0 0 z VV /1 4 4~J1~ 6 [29] (0.0.2) fiE 1f o OOO ~ o O i 5 4~Ji~ 12 [37] (4,4,4) fA o, 0 6 ~ yp 6 24 [49 ...
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We study the optical properties of hydrogen passivated silicon, germanium and mixed Ge/Si core/shell quantum dots (QDs) using high accuracy Density Functional Theory (DFT) and time-dependent DFT (TDFT). We employ the hybrid DFT functional... more
We study the optical properties of hydrogen passivated silicon, germanium and mixed Ge/Si core/shell quantum dots (QDs) using high accuracy Density Functional Theory (DFT) and time-dependent DFT (TDFT). We employ the hybrid DFT functional of Becke, Lee, Yang and Parr (B3LYP) in combination with good quality basis sets. As we have shown in our previous work, this combination is an accurate and computationally efficient way for such calculations. The mixed quantum dots, as would be expected, are more versatile and offer more possibilities for band gap engineering, with gap values (electronic and optical) between those of the corresponding Si and Ge dots. Our results support the quantum confinement theory for all three types of QDs.
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ABSTRACT
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In previous work1,2, we have obtained an understanding of the role of hydrogen in amorphous silicon (a-Si) performing first principles calculations of the electronic and transport properties of a-Si:H using the coherent potential... more
In previous work1,2, we have obtained an understanding of the role of hydrogen in amorphous silicon (a-Si) performing first principles calculations of the electronic and transport properties of a-Si:H using the coherent potential approximation (CPA).
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In contrast to alternant nanographenes (NGRs), in non-alternant NGRs no “sublattice structure” can be defined associated with significant conceptual and computational simplifications. This leads to some fundamental differences between the... more
In contrast to alternant nanographenes (NGRs), in non-alternant NGRs no “sublattice structure” can be defined associated with significant conceptual and computational simplifications. This leads to some fundamental differences between the two. We uncover here the broken electron-hole symmetry in non-alternant NGRs as one fundamental difference closely related to distorted Dirac points (cones) and their diradical open-shell character. We also show by higher level calculations beyond common DFT that the alternant series of peri-acenes (bisanthene, peri-tetracene, peri-pentacene, … etc.), contrary to opposite reports in the literature, have clearly closed singlet ground states, in contrast to their non-alternant isomers based on Stone-Wales (SW) defects. This is experimentally supported by sub-molecularly resolved STM images. The misconceptions in the literature are due to insufficient correlation. For non-alternant NGRs/GNRs with antiaromatic rings the driving force for open-shell sta...
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Research Interests: Physics, Materials Science, Chemical Physics, Multidisciplinary, Optical Waveguides, and 15 moreQuantum Dots, DFT calculation, Density Functional Theory, Electronic properties, Oxygen, Chemical Bonds, Binding Energy, Calculations, Nanocrystals, Molecular Orbitals, DFT Calculations, Nanocrystal, Semiconductor Quantum Dots, Bins, and hydrogen terminated silicon
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Using state of the art time-dependent density functional theory and multireference second-order perturbation theory, we have accurately calculated (within 0.3 eV) in real space the optical gap of small silicon quantum dots, with diameters... more
Using state of the art time-dependent density functional theory and multireference second-order perturbation theory, we have accurately calculated (within 0.3 eV) in real space the optical gap of small silicon quantum dots, with diameters up to 25 Å. Our results, which support the quantum confinement hypothesis, are in excellent agreement with recent and earlier experimental data on oxygen-free samples and the conclusions of Wilcoxon et al. [Phys. Rev. B 60, 2704 (1999)]. We have found that the diameter of the smallest dot, which could emit photoluminescence in the visible region of the spectrum, is around 22 Å. Our work can resolve existing controversies and bridge diverse experimental and theoretical results.
It is demonstrated that in wider armchair graphene nanoribbons (AGNRs), and in particular the 13-AGNRs, there is a critical length Lc where two (or more) pairs of topological end-stats of opposite parities and nearly equal energies can... more
It is demonstrated that in wider armchair graphene nanoribbons (AGNRs), and in particular the 13-AGNRs, there is a critical length Lc where two (or more) pairs of topological end-stats of opposite parities and nearly equal energies can develop in a form of a phase transition, giving rise to a sudden jump in the nominal conductivity by a factor of about 2.5, which is interpreted as a signature of constructive quantum interference (CQI). The “phase transition” is also associated with an abrupt increase in the energy gap(s) of about the same magnitude. Such unusual CQI, in which the role of “anchors” can be considered to be effectively played by the zigzag-end region, persists for lengths Lc ≈ 40Å up to L≈ 80Å and is unique for the 13-AGNRs, in contrast to other wide AGNRs (11-, 15-) studied here, in which the “phase transition” occurs without CQI. This is attributed to the aromaticity of 13-AGNRs which is characterized by “migration of sextets”, based on the resonance of two different...
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We present a comprehensive and integrated model-independent ab initio study of the structural, cohesive, electronic, and optical properties of silicon quantum dots of various morphologies and sizes in the framework of all-electron static... more
We present a comprehensive and integrated model-independent ab initio study of the structural, cohesive, electronic, and optical properties of silicon quantum dots of various morphologies and sizes in the framework of all-electron static and time-dependent density functional theory (DFT, TDFT), using the well-tested B3LYP and other properly chosen functional(s). Our raw ab initio results for all these properties for hydrogen passivated nanocrystals of various growth models and sizes from 1 to 32 Angstroms, are subsequently fitted, using power-law dependence with judicially selected exponents, based on dimensional and other plausibility arguments. As a result, we can reproduce with excellent accuracy not only known experimental and well-tested theoretical results in the regions of overlap, but we can also extrapolate successfully all the way to infinity, reproducing the band gap of crystalline silicon with almost chemical accuracy as well as the cohesive energy of the infinite crysta...
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By bridging graphene and benzene through a well-defined sequence of polycyclic aromatic hydrocarbons and their inherent shell structure, J. Phys. Chem. C, 2018, 122, 17526, it is shown that graphene is actually a coherent arrangement of... more
By bridging graphene and benzene through a well-defined sequence of polycyclic aromatic hydrocarbons and their inherent shell structure, J. Phys. Chem. C, 2018, 122, 17526, it is shown that graphene is actually a coherent arrangement of interwoven benzene molecules, which are coordinated by aromaticity, shell structure and topology, all interrelated. The exotic properties of graphene are in fact macroscopic manifestations of aromaticity. At the molecular level this is revealed as a dynamical flipping of the atomic pz-orbitals belonging to different sublattices, leading to a dynamical interchange between aromatic and non-aromatic rings, associated with frontier occupied-unoccupied orbitals or valence-conduction bands interchange with opposite parities, in full agreement with many-body and topological insulators theory. Such dynamical interchange or coupling is driven by inversion symmetry-breaking, activated through geometrical frustration between sublattice and space group symmetry....
The successful “molecular” description of graphene through a sequence of growing size polycyclic aromatic hydrocarbons (PAHs), not only serves as a bridge between the molecular and crystalline nature of graphene, but also at the same time... more
The successful “molecular” description of graphene through a sequence of growing size polycyclic aromatic hydrocarbons (PAHs), not only serves as a bridge between the molecular and crystalline nature of graphene, but also at the same time links and interrelates in a transparent and comprehensive way the electronic and aromatic properties of benzene with those of graphene, revealing that graphene like benzene is a resonance structure, “resonating” between two complementary aromaticity patterns [1-2]. Thus, graphene is the prototypical aromatic “crystal” in full analogy to benzene which is the prototypical aromatic molecule. The two are connected through aromaticity, which is proven a very useful, simple, and transparent “everyday” tool, rather than a
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Silicon nanoparticles, which are the building blocks of porous (nano-porous and meso-porous) silicon, are characterized by biocompatibility, biodegradability, low toxicity and solubility; and they exhibit extraordinary qualities for... more
Silicon nanoparticles, which are the building blocks of porous (nano-porous and meso-porous) silicon, are characterized by biocompatibility, biodegradability, low toxicity and solubility; and they exhibit extraordinary qualities for biochemical applications for controlled drug delivery and for tissue engineering. For an efficient design and functionality of such complex systems a fundamental understanding of their biochemical properties and interactions is indispensable. The present study aims at the fundamental ab initio description and understanding at the molecular level of such interactions, which also constitute a first basic step towards a bottom up multiscalar approach for the drug delivering process, based on ab initio density functional theory (DFT). At this level our study is twofold: (1) To study the chemical and biochemical properties of silicon nanocrystals, which are the building blocks of nanoand mesoporous silicon, already used as drug carriers; and (2) to explore th...
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Rationalization of energy gaps of atomically precise AGNRs, “bulk” (ΔΕac) or “zigzag-end” (ΔΕzz), could be challenging and controversial concerning their magnitude, origin, substrate influence (ΔΕsb), and spin-polarization, among others.... more
Rationalization of energy gaps of atomically precise AGNRs, “bulk” (ΔΕac) or “zigzag-end” (ΔΕzz), could be challenging and controversial concerning their magnitude, origin, substrate influence (ΔΕsb), and spin-polarization, among others. Hereby, a simple self-consistent and “economical” interpretation is presented, based on “appropriate” DFT (and TDDFT) calculations, general symmetry principles, and plausibility arguments, which is fully consistent with current experimental measurements for 5-, 7-, and 9-AGNRs within less than 1%, although at variance with some prevailing views or interpretations for ΔΕac, ΔΕzz, and ΔΕsb. Thus, an excellent agreement between experiment and theory emerges, provided some established stereotypes are reconsidered and/or abandoned. The primary source of discrepancies is the finite length of AGNRs together with inversion-symmetry conflict and topological end/edge states, which invariably mix with other “bulk” states making their unambiguous detection/dist...
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We show that very narrow armchair graphene nanoribbons (AGNRs) of length L and width of 2 zigzag-rings undergo a metal-insulator-like transition at a critical length of 10nm, where the energy gap drops rather abruptly, and the... more
We show that very narrow armchair graphene nanoribbons (AGNRs) of length L and width of 2 zigzag-rings undergo a metal-insulator-like transition at a critical length of 10nm, where the energy gap drops rather abruptly, and the conductivity, estimated, through an invoked computational scheme, rises almost discontinuously to a value between that of a perfect quasi one-dimensional system, and the nominal minimum conductivity of graphene. At this length, the aromatic and non-aromatic rings are interchanged, and sharp peaks appear in the density of states around the Fermi level, suggesting metallic-like behaviour. Such peaks linked to edge states at the Dirac point(s) coincide with the charge-neutrality point(s), associated with the minimum conductivity of graphene. Thus, we have an uncommon combination of interrelated “short-long”, “core-edge,” topological-aromatic transition(s) due to strong quantum confinement, driven by inversion symmetry conflict. The bandgap decreases with the 2/3 ...
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,... more
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 pz-orbitals, playing the role of pseudospin or “qubits”. This renders graphene resonance structure, “resonating” between two complementary aromaticity patterns, involving 2k, k→∞ kekulé type of resonances resulting in “robust electronic coherence”, with 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 accord with topological-insulator and many-body theory. The “average picture” converges to the usual band structure with two aromatic π-electrons per ring, and the finge...
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... F: Metal Phys. 3 39-54 Gaspari G D and Gyorffy BL 1972 P17ps. Rer. Lett 28 801-5 Hedin L andLundqvist B I 1971 J . Phps. C: %[id Sr. Phys. 4 206683 John W 1973 J. Phys. F; Metal Phys. 3 L23-33 Koelling DD and Harmon B N 1977 J. Phys.... more
... F: Metal Phys. 3 39-54 Gaspari G D and Gyorffy BL 1972 P17ps. Rer. Lett 28 801-5 Hedin L andLundqvist B I 1971 J . Phps. C: %[id Sr. Phys. 4 206683 John W 1973 J. Phys. F; Metal Phys. 3 L23-33 Koelling DD and Harmon B N 1977 J. Phys. C: Solid St. Phys. ...