83 research outputs found
Modifications of the hydrogen bond network of liquid water in a cylindrical SiO_2 pore
We present results of molecular dynamics simulations of water confined in a
silica pore. A cylindrical cavity is created inside a vitreous silica cell with
geometry and size similar to the pores of real Vycor glass. The simulations are
performed at different hydration levels. At all hydration levels water adsorbs
strongly on the Vycor surface; a double layer structure is evident at higher
hydrations. At almost full hydration the modifications of the
confinement-induced site-site pair distribution functions are in qualitative
agreement with neutron diffraction experiment. A decrease in the number of
hydrogen bonds between water molecules is observed along the pore radius, due
to the tendency of the molecules close to the substrate to form hydrogen-bonds
with the hydrophilic pore surface. As a consequence we observe a substrate
induced distortion of the H-bond tetrahedral network of water molecules in the
regions close to the surface.Comment: Talk presented at "Physics of Liquids: Foundations, Highlights,
Challenge", Murau Sept. 1998. To appear in J. Mol. Li
Wick's theorem for q-deformed boson operators
In this paper combinatorial aspects of normal ordering arbitrary words in the
creation and annihilation operators of the q-deformed boson are discussed. In
particular, it is shown how by introducing appropriate q-weights for the
associated ``Feynman diagrams'' the normally ordered form of a general
expression in the creation and annihilation operators can be written as a sum
over all q-weighted Feynman diagrams, representing Wick's theorem in the
present context.Comment: 9 page
Operator Method for Nonperturbative Calculation of the Thermodynamic Values in Quantum Statistics. Diatomic Molecular Gas
Operator method and cumulant expansion are used for nonperturbative
calculation of the partition function and the free energy in quantum
statistics. It is shown for Boltzmann diatomic molecular gas with some model
intermolecular potentials that the zeroth order approximation of the proposed
method interpolates the thermodynamic values with rather good accuracy in the
entire range of both the Hamiltonian parameters and temperature. The systematic
procedure for calculation of the corrections to the zeroth order approximation
is also considered.Comment: 22 pages, 7 Postscript figures, accepted for publication in Journal
of Physics
Charge-density-wave instability in the Holstein model with quartic anharmonic phonons
The molecular-crystal model, that describes a one-dimensional electron gas
interacting with quartic anharmonic lattice vibrations, offers great potentials
in the mapping of a relatively wide range of low-dimensional fermion systems
coupled to optical phonons onto quantum liquids with retarded interactions.
Following a non-perturbative approach involving non-Gaussian partial functional
integrations of lattice degrees of freedom, the exact expression of the
phonon-mediated two-electron action for this model is derived. With the help of
Hubbard-Stratonovich transformation the charge-density-wave instability is
examined in the sequel, with particular emphasis on the effect of the quartic
anharmonic phonons on the charge-density-wave transition temperature.Comment: 12 pages, 3 figure
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