22 research outputs found
The Phase Diagram of an Anisotropic Potts Model
A study is made of an anisotropic Potts model in three dimensions where the
coupling depends on both the Potts state on each site but also the direction of
the bond between them using both analytical and numerical methods. The phase
diagram is mapped out for all values of the exchange interactions. Six distinct
phases are identified. Monte Carlo simulations have been used to obtain the
order parameter and the values for the energy and entropy in the ground state
and also the transition temperatures. Excellent agreement is found between the
simulated and analytic results. We find one region where there are two phase
transitions with the lines meeting in a triple point. The orbital ordering that
occurs in occurs as one of the ordered phases.Comment: 30 pages, 19 figures, one tabl
Thermodynamic characteristics of the classical n-vector magnetic model in three dimensions
The method of calculating the free energy and thermodynamic characteristics
of the classical n-vector three-dimensional (3D) magnetic model at the
microscopic level without any adjustable parameters is proposed. Mathematical
description is perfomed using the collective variables (CV) method in the
framework of the model approximation. The exponentially decreasing
function of the distance between the particles situated at the N sites of a
simple cubic lattice is used as the interaction potential. Explicit and
rigorous analytical expressions for entropy,internal energy, specific heat near
the phase transition point as functions of the temperature are obtained. The
dependence of the amplitudes of the thermodynamic characteristics of the system
for and on the microscopic parameters of the interaction
potential are studied for the cases and . The obtained
results provide the basis for accurate analysis of the critical behaviour in
three dimensions including the nonuniversal characteristics of the system.Comment: 25 pages, 5 figure
Hydrogen bonds: relation between lengths and electron densities at bond critical points
The electron densities for a number of molecules with either inter- or
intra-molecular hydrogen bonds are analyzed using the theory of atoms in
molecules. The levels of theory used include second order Møller Plesset
and density functional methods. The molecules investigated ranges from small
molecules/ions to an alanine octa-peptide. The hydrogen bond length, BL,
varies from 1.15 to 3.01 Å and , the electron density at the
bond critical point, spans the interval 0.0033 to 0.168 (au). We find that
the data may be represented by the relation
), where A and B are empirical constants. The relation is
compared to a similar relation derived from solid state experiments. Since
the values are related to the bond strengths, this general
relationship may useful for hydrogen bond studies
The rotation of NO3− as a probe of molecular ion - water interactions
The hydration dynamics of aqueous nitrate, NO3−(aq), is studied by 2D-IR spectroscopy, UV-IR- and UV-UV transient absorption spectroscopy. The experimental results are compared to Car-Parinello molecular dynamics (MD) simulations. The 2D-IR measurements and MD simulations of the non-degenerate asymmetric stretch vibrations of nitrate reveal an intermodal energy exchange occurring on a 0.2 ps time scale related to hydrogen bond fluctuations. The transient absorption measurements find that the nitrate ions rotate in 2 ps. The MD simulations indicate that the ion rotation is associated with the formation of new hydrogen bonds. The 2 ps rotation time thus indicates that the hydration shell of aqueous nitrate is rather labile
The rotation of NO
The hydration dynamics of aqueous nitrate, NO3−(aq), is studied by 2D-IR spectroscopy, UV-IR- and UV-UV transient absorption spectroscopy. The experimental results are compared to Car-Parinello molecular dynamics (MD) simulations. The 2D-IR measurements and MD simulations of the non-degenerate asymmetric stretch vibrations of nitrate reveal an intermodal energy exchange occurring on a 0.2 ps time scale related to hydrogen bond fluctuations. The transient absorption measurements find that the nitrate ions rotate in 2 ps. The MD simulations indicate that the ion rotation is associated with the formation of new hydrogen bonds. The 2 ps rotation time thus indicates that the hydration shell of aqueous nitrate is rather labile
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Latest results from the European Mars simulation wind tunnel facility
The Mars Simulation wind tunnel at Aarhus University is a unique ESA and EU supported simulation facility designed for studies of the Martian environment. Specifically it is capable of recreating the key physical parameters such as temperature, pressure (composition), wind flow and importantly the suspension of dust particulates. This facility is available to the scientific community for collaborative research. Preliminary results from the first year of facility operations will be presented