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Publications during 1996

Article Reference A First Principles Investigation of the Structure of a Bacteriochlorophyll Crystal
Article Reference Ab initio Molecular Dynamics Simulation of Laser Melting of Silicon
The method of ab initio molecular dynamics, based on finite temperature density functional theory, is used to simulate laser heating of crystal silicon. We have found that a high concentration of excited electrons dramatically weakens the covalent bond. As a result, the system undergoes a melting transition to a metallic state. In contrast to ordinary liquid silicon, the new liquid is characterized by a high coordination number and a strong reduction of covalent bonding effects.
Article Reference Ab initio molecular dynamics simulation of liquid water: Comparison of three gradient-corrected density functionals
Article Reference Ab initio path integral molecular dynamics: Basic ideas
Article Reference Ab-initio simulation of phase transformations under pressure
Article Reference Anisotropic a-C:H from Compression of Polyacetylene
We have simulated the transformation of crystalline trans-polyacetylene into a-C:H under pressure by constant pressure ab initio molecular dynamics. Polyacetylene undergoes a gradual saturation of C-C bonds via chain interlinks, ending up at ∼50GPa with a-C:H containing 80\% sp3 carbon atoms. The sp2-sp3 conversion is irreversible and does not reverse by returning to zero pressure. The final a-C:H is a wide gap insulator and, at variance with the conventionally generated a-C:H, is highly anisotropic keeping some memory of the original polyacetylene chain axis.
Article Reference Anomalous electronic behaviour of Na superfullerides: theory and experiment
Article Reference Dynamics, Spin Fluctuations, and Bonding in Liquid Silicon
We present a large-scale molecular dynamics simulation of liquid silicon close to the melting point. We find that inclusion of spin has appreciable effects on the description of the bond breaking and forming processes. This improves the description of the structure, significantly modifies the dynamics, and suggests a possible explanation of measured anomalies in properties just above the melting point.
Article Reference Effects of Doping on the Vibrational Properties of C60 from First Principles: K6C60
Ab initio calculations of the phonon spectrum of K6C60 are presented, based on the local-density approximation of density-functional theory and on linear response theory. The effects of doping on frequencies and infrared intensities are identified and their physical origin discussed in detail for optically allowed modes. Whereas structural relaxation is primarily responsible for the frequency changes, and especially for the softening of tangential modes, the change in infrared relative intensities is a direct consequence of the electron transfer. The potassium vibrations are found to lie within 60-100 cm-1 and are well decoupled from C60 intramolecular modes.
Article Reference Efficient and general algorithms for path integral Car-Parrinello molecular dynamics
Article Reference Electronic structure, screening, and charging effects at a metal/organic tunneling junction: A first-principles study
By ab initio simulation in the density-functional local-density approximation, we study the structural and electronic properties of a monolayer of Pd-porphyrin and perylene on a Au(111) slab, and we investigate the response of this interface to an external electrostatic field. Our computation closely mimics the metal/organic junction investigated experimentally by Fischer et al. [Europhys. Lett. 28, 129 (1994)] that has been shown to display rectifying behavior and charging effects associated with molecular conduction via single-electron tunneling. The ab initio model allows us to determine the conditions for molecular tunneling and to investigate their dependence on structural and chemical parameters. Moreover, it provides a direct view of screening at the molecular level. © 1996 The American Physical Society.
Article Reference Freedom and Constraints of a Metal Atom Encapsulated in Fullerene Cages
Ab initio molecular dynamics applied to fullerenes with La and Y inside reveals an unforeseen picture of these fascinating host-guest complexes and answers long-standing basic questions on the metal-cage interaction. Adsorption sites of the metal atom, nature, and time scale of its motion are identified. They depend on the cage structure in a simple way, applicable to any fullerenelike host with electron donors inside. Results for (La, Y)@C82 are consistent with experiment and suggest its potential as electronic molecular switch. Light is shed onto the strikingly poorer stability of (La, Y)@C80.
Article Reference Geometry and electronic structure of porphyrins and porphyrazines
We present a theoretical investigation based on density functional theory of the geometry and electronic structure of free base porphyrin and porphyrazine and their magnesium and palladium derivatives. The calculations are performed within the local density approximation and make use of pseudopotentials and plane waves basis functions. Our results are in good agreement with experiment and other ab initio methods using localized basis sets. A comparison between the different molecules is made.
Article Reference Hot electrons and the approach to metallic behaviour in Kx(KCl)1 − x
Article Reference Ionic Diffusion in a Ternary Superionic Conductor: An Ab Initio Molecular Dynamics Study
We present Car-Parrinello molecular dynamics simulations of a novel superionic conductor, Li2-2xMg1+xSi (x∼0.06), at different temperatures. The calculations clarify the nature of the ionic conduction and lead to the prediction of the first inorganic magnesium superionic conductor. In fact, both lithium and magnesium are found to act as charge carriers. The diffusion is fast and can be described as vacancy migration through directed jumps. The calculated diffusion constants for lithium are consistent with recent electrochemical measurements.
Article Reference Isolation, Structure, and Electronic Calculations of the Heterofullerene Salt K6C59N
Article Reference Microscopic Structure of Tetrahedral Amorphous Carbon
Simulations are reported of a highly tetrahedral amorphous carbon network at a density of 3 g/cc using Car-Parrinello first principles molecular dynamics. The simulated structure consists of 65\% fourfold and 35\% threefold coordinated carbon sites, in good agreement with experiment. The structure is also in good agreement with recent neutron diffraction data. An unexpected observation was the presence of small carbon rings in the structure containing as few as three atoms. These carbon triangles and quadrilaterals, which resemble the organic compounds cyclopropane and cyclobutane, give the network a topology unique among tetrahedrally bonded amorphous materials.
Article Reference New High-Pressure Phase of Ice
Based on ab initio constant pressure molecular dynamics simulation, we predict a new phase transition in ice from ice X to a new phase (ice XI) at ∼3Mbar and room temperature. Ice XI preserves the symmetric hydrogen bond and is a wide gap insulator up to and beyond 7 Mbar. This new phase is stable up to 2000 K at 4 Mbar, where the system shows large protonic diffusion.
Article Reference Nonempirical Calculations of a Hydrated RNA Duplex
Article Reference Nonmetal-metal transition in metal molten-salt solutions
The method of ab initio molecular dynamics, based on finite-temperature density-functional theory, is used to study the nonmetal-metal transition in two different metal molten-salt solutions, Kx(KCl)1-x and Nax(NaBr)1-x. As the excess metal concentration is increased the electronic density becomes delocalized and percolating conducting paths are formed, making a significant dc electrical conductivity possible. This marks the onset of the metallic regime. By calculating several electronic and structural properties, remarkable differences between the two solutions are observed. The anomalous behavior of Nax(NaBr)1-x, typical of all the Na-NaX solutions, is found to be related to the strong attractive interaction between the sodium ions and the excess electrons. © 1996 The American Physical Society.
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