8 research outputs found
Theory for structure and bulk-modulus determination
A new method for direct evaluation of both crystalline structure, bulk
modulus B_0, and bulk-modulus pressure derivative B'_0 of solid materials with
complex crystal structures is presented. The explicit and exact results
presented here permit a multidimensional polynomial fit of the total energy as
a function of all relevant structure parameters to simultaneously determine the
equilibrium configuration and the elastic properties. The method allows for
inclusion of general (internal) structure parameters, e.g., bond lengths and
angles within the unit cell, on an equal footing with the unit-cell lattice
parameters. The method is illustrated by the calculation of B_0 and B'_0 for a
few selected materials with multiple structure parameters for which data is
obtained by using first-principles density functional theory.Comment: 7 pages, 2 figures, submitted to Phys. Rev.
Adsorption of Methanol on Aluminum Oxide: A Density Functional Study
Theoretical calculations based on density functional theory have made
significant contributions to our understanding of metal oxides, their surfaces,
and the binding of molecules at these surfaces. In this paper we investigate
the binding of methanol at the alpha-Al2O3(0001) surface using first-principles
density functional theory. We calculate the molecular adsorption energy of
methanol to be E^g_ads=1.03 eV/molecule. Taking the methanol-methanol
interaction into account, we obtain the adsorption energy E_ads=1.01
eV/molecule. Our calculations indicate that methanol adsorbs chemically by
donating electron charge from the methanol oxygen to the surface aluminum. We
find that the surface atomic structure changes upon adsorption, most notably
the spacing between the outermost Al and O layers changes from 0.11 Angstrom to
0.33 Angstrom.Comment: 11 pages, 3 figures, 1 tabl
A Harris-type van der Waals density functional scheme
Large biomolecular systems, whose function may involve thousands of atoms,
cannot easily be addressed with parameter-free density functional theory (DFT)
calculations. Until recently a central problem was that such systems possess an
inherent sparseness, that is, they are formed from components that are mutually
separated by low-electron-density regions where dispersive forces contribute
significantly to the cohesion and behavior. The introduction of, for example,
the van der Waals density functional (vdW-DF) method [PRL 92, 246401 (2004)]
has addressed part of this sparse-matter system challenge. However, while a
vdW-DF study is often as computationally efficient as a study performed in the
generalized gradient approximation, the scope of large-sparse-matter DFT is
still limited by computer time and memory. It is costly to self-consistently
determine the electron wavefunctions and hence the kinetic-energy repulsion. In
this paper we propose and evaluate an adaption of the Harris scheme [PRB 31,
1770 (1985)]. This is done to speed up non-selfconsistent vdW-DF studies of
molecular-system interaction energies. Also, the Harris-type analysis
establishes a formal link between dispersion-interaction effects on the
effective potential for electron dynamics and the impact of including
selfconsistency in vdW-DF calculations [PRB 76, 125112 (2007)].Comment: 15 page
Diffusion and Weak Turbulence
Contents Introduction 3 1 Diffusion in Surface Waves 6 1.1 The Faraday Experiment . . . . . . . . . . . . . . . . . . . . . . . 6 1.2 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.3 Notation and Tools . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.4 Surface Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2 Relative Particle Motion 15 2.1 Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.2 Relative Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.3 Moments of the Relative Diffusivity . . . . . . . . . . . . . . . . . 19 3 Theory of Weak Turbulence 32 3.1 Hamiltonian Formalism . . . . . . . . . . . . . . . . . . . . . . . . 33 3.2 Kinetic Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.2.1 Derivation of Three-Wave Kinetic Equation . . . . . . . . 36 3.2.2 The Four-Wave Kinetic Equation . . . . . . . . . . . . . . 40 3.3 Application to Surface Waves . . . . .
Physica A 239 (1997) 314
On the surface of a vertically oscillated fluid, capillary waves with a clearly discernible wavelength are formed if the amplitude of the oscillations exceeds a critical value. Particles sprinkled on the fluid surface are experimentally found to move in an almost Brownian motion when measured over distances larger than . We extend earlier studies of the diffusivity to length scales ranging from 0:1 to 10. We observe a cross-over in the diffusive motion from a strongly anomalous diffusion below , to a motion that is closer to being Brownian above . Our observations show that the particle motion is well described by an amplitude-independent fractional Brownian motion, effective at sizes less than , convoluted with an amplitude-dependent fractional Brownian motion, effective on all length scales smaller than the system size