166 research outputs found
Longitudinal wavevector- and frequency-dependent dielectric constant of the TIP4P water model
A computer adapted theory for self-consistent calculations of the wavevector-
and frequency-dependent dielectric constant for interaction site models of
polar systems is proposed. A longitudinal component of the dielectric constant
is evaluated for the TIP4P water model in a very wide scale of wavenumbers and
frequencies using molecular dynamics simulations. It is shown that values for
the dielectric permittivity, calculated within the exact interaction site
description, differ in a characteristic way from those obtained by the point
dipole approximation which is usually used in computer experiment. It is also
shown that the libration oscillations, existing in the shape of longitudinal
time-dependent polarization fluctuations at small and intermediate wavevector
values, vanish however for bigger wavenumbers. A comparison between the
wavevector and frequency behaviour of the dielectric constant for the TIP4P
water and the Stockmayer model is made. The static screening of external
charges and damping of longitudinal electric excitations in water are
considered as well. A special investigation is devoted to the time dependence
of dielectric quantities in the free motion regime.Comment: 21 pages, 7 figure
Optimized Verlet-like algorithms for molecular dynamics simulations
New explicit velocity- and position-Verlet-like algorithms of the second
order are proposed to integrate the equations of motion in many-body systems.
The algorithms are derived on the basis of an extended decomposition scheme at
the presence of a free parameter. The nonzero value for this parameter is
obtained by reducing the influence of truncated terms to a minimum. As a
result, the new algorithms appear to be more efficient than the original Verlet
versions which correspond to a particular case when the introduced parameter is
equal to zero. Like the original versions, the proposed counterparts are
symplectic and time reversible, but lead to an improved accuracy in the
generated solutions at the same overall computational costs. The advantages of
the new algorithms are demonstrated in molecular dynamics simulations of a
Lennard-Jones fluid.Comment: 5 pages, 2 figures; submitted to Phys. Rev.
Optimized Forest-Ruth- and Suzuki-like algorithms for integration of motion in many-body systems
An approach is proposed to improve the efficiency of fourth-order algorithms
for numerical integration of the equations of motion in molecular dynamics
simulations. The approach is based on an extension of the decomposition scheme
by introducing extra evolution subpropagators. The extended set of parameters
of the integration is then determined by reducing the norm of truncation terms
to a minimum. In such a way, we derive new explicit symplectic Forest-Ruth- and
Suzuki-like integrators and present them in time-reversible velocity and
position forms. It is proven that these optimized integrators lead to the best
accuracy in the calculations at the same computational cost among all possible
algorithms of the fourth order from a given decomposition class. It is shown
also that the Forest-Ruth-like algorithms, which are based on direct
decomposition of exponential propagators, provide better optimization than
their Suzuki-like counterparts which represent compositions of second-order
schemes. In particular, using our optimized Forest-Ruth-like algorithms allows
us to increase the efficiency of the computations more than in ten times with
respect to that of the original integrator by Forest and Ruth, and
approximately in five times with respect to Suzuki's approach. The theoretical
predictions are confirmed in molecular dynamics simulations of a Lennard-Jones
fluid. A special case of the optimization of the proposed Forest-Ruth-like
algorithms to celestial mechanics simulations is considered as well.Comment: 12 pages, 3 figures; submitted to Computer Physics Communication
Symplectic integrators for classical spin systems
We suggest a numerical integration procedure for solving the equations of
motion of certain classical spin systems which preserves the underlying
symplectic structure of the phase space. Such symplectic integrators have been
successfully utilized for other Hamiltonian systems, e. g. for molecular
dynamics or non-linear wave equations. Our procedure rests on a decomposition
of the spin Hamiltonian into a sum of two completely integrable Hamiltonians
and on the corresponding Lie-Trotter decomposition of the time evolution
operator. In order to make this method widely applicable we provide a large
class of integrable spin systems whose time evolution consists of a sequence of
rotations about fixed axes. We test the proposed symplectic integrator for
small spin systems, including the model of a recently synthesized magnetic
molecule, and compare the results for variants of different order
A consistent description of kinetics and hydrodynamics of systems of interacting particles by means of the nonequilibrium statistical operator method
A statistical approach to a self-consistent description of kinetic and
hydrodynamic processes in systems of interacting particles is formulated on the
basis of the nonequilibrium statistical operator method by D.N.Zubarev. It is
shown how to obtain the kinetic equation of the revised Enskog theory for a
hard sphere model, the kinetic equations for multistep potentials of
interaction and the Enskog-Landau kinetic equation for a system of charged hard
spheres. The BBGKY hierarchy is analyzed on the basis of modified group
expansions. Generalized transport equations are obtained in view of a
self-consistent description of kinetics and hydrodynamics. Time correlation
functions, spectra of collective excitations and generalized transport
coefficients are investigated in the case of weakly nonequilibrium systems of
interacting particles.Comment: 64 LaTeX2e pages, 1 figure, special sty-files, additional font
Algorithm for numerical integration of the rigid-body equations of motion
A new algorithm for numerical integration of the rigid-body equations of
motion is proposed. The algorithm uses the leapfrog scheme and the quantities
involved are angular velocities and orientational variables which can be
expressed in terms of either principal axes or quaternions. Due to specific
features of the algorithm, orthonormality and unit norms of the orientational
variables are integrals of motion, despite an approximate character of the
produced trajectories. It is shown that the method presented appears to be the
most efficient among all known algorithms of such a kind.Comment: 4 pages, 1 figur
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