61,347 research outputs found
From ab initio quantum chemistry to molecular dynamics: The delicate case of hydrogen bonding in ammonia
The ammonia dimer (NH3)2 has been investigated using high--level ab initio
quantum chemistry methods and density functional theory (DFT). The structure
and energetics of important isomers is obtained to unprecedented accuracy
without resorting to experiment. The global minimum of eclipsed C_s symmetry is
characterized by a significantly bent hydrogen bond which deviates from
linearity by about 20 degrees. In addition, the so-called cyclic C_{2h}
structure is extremely close in energy on an overall flat potential energy
surface. It is demonstrated that none of the currently available (GGA,
meta--GGA, and hybrid) density functionals satisfactorily describe the
structure and relative energies of this nonlinear hydrogen bond. We present a
novel density functional, HCTH/407+, designed to describe this sort of hydrogen
bond quantitatively on the level of the dimer, contrary to e.g. the widely used
BLYP functional. This improved functional is employed in Car-Parrinello ab
initio molecular dynamics simulations of liquid ammonia to judge its
performance in describing the associated liquid. Both the HCTH/407+ and BLYP
functionals describe the properties of the liquid well as judged by analysis of
radial distribution functions, hydrogen bonding structure and dynamics,
translational diffusion, and orientational relaxation processes. It is
demonstrated that the solvation shell of the ammonia molecule in the liquid
phase is dominated by steric packing effects and not so much by directional
hydrogen bonding interactions. In addition, the propensity of ammonia molecules
to form bifurcated and multifurcated hydrogen bonds in the liquid phase is
found to be negligibly small.Comment: Journal of Chemical Physics, in press (305335JCP
Electroelasticity of Charged Black Branes
We present the first-order corrected dynamics of fluid branes carrying
higher-form charge by obtaining the general form of their equations of motion
to pole-dipole order. Assuming linear response theory, we characterize the
corresponding effective theory of stationary bent charged (an)isotropic fluid
branes in terms of two sets of response coefficients, the Young modulus and the
piezoelectric moduli. We subsequently find large classes of examples in gravity
of this effective theory, by constructing stationary strained charged black
brane solutions to first order in a derivative expansion. Using solution
generating techniques and bent neutral black branes as a seed solution, we
obtain a class of charged black brane geometries carrying smeared Maxwell
charge in Einstein-Maxwell-dilaton gravity. In the specific case of
ten-dimensional space-time we furthermore use T-duality to generate bent black
branes with higher-form charge, including smeared D-branes of type II string
theory. By subsequently measuring the bending moment and the electric dipole
moment which these geometries acquire due to the strain, we uncover that their
form is captured by classical electroelasticity theory. In particular, we find
that the Young modulus and the piezoelectric moduli of our strained charged
black brane solutions are parameterized by a total of 4 response coefficients,
both for the isotropic as well as anisotropic cases.Comment: v2: 40pp; typos fixe
Towards an effective potential for the monomer, dimer, hexamer, solid and liquid forms of hydrogen fluoride
We present an attempt to build up a new two-body effective potential for
hydrogen fluoride, fitted to theoretical and experimental data relevant not
only to the gas and liquid phases, but also to the crystal. The model is simple
enough to be used in Molecular Dynamics and Monte Carlo simulations. The
potential consists of: a) an intra-molecular contribution, allowing for
variations of the molecular length, plus b) an inter-molecular part, with three
charged sites on each monomer and a Buckingham "exp-6" interaction between
fluorines. The model is able to reproduce a significant number of observables
on the monomer, dimer, hexamer, solid and liquid forms of HF. The shortcomings
of the model are pointed out and possible improvements are finally discussed.Comment: LaTeX, 24 pages, 2 figures. For related papers see also
http://www.chim.unifi.it:8080/~valle
A Generalization of APN Functions for Odd Characteristic
Almost perfect nonlinear (APN) functions on finite fields of characteristic
two have been studied by many researchers. Such functions have useful
properties and applications in cryptography, finite geometries and so on.
However APN functions on finite fields of odd characteristic do not satisfy
desired properties. In this paper, we modify the definition of APN function in
the case of odd characteristic, and study its properties
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