22 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
On the numerical integration of motion for rigid polyatomics: The modified quaternion approach
A revised version of the quaternion approach for numerical integration of the
equations of motion for rigid polyatomic molecules is proposed. The modified
approach is based on a formulation of the quaternion dynamics with constraints.
This allows to resolve the rigidity problem rigorously using constraint forces.
It is shown that the procedure for preservation of molecular rigidity can be
realized particularly simply within the Verlet algorithm in velocity form. We
demonstrate that the presented method leads to an improved numerical stability
with respect to the usual quaternion rescaling scheme and it is roughly as good
as the cumbersome atomic-constraint technique.Comment: 14 pages, 2 figure
Ewald summation technique for interaction site models of polar fluids
A computer adapted fluctuation formula for the calculation of the wavevector-
and frequency-dependent dielectric permittivity for interaction site models of
polar fluids within the Ewald summation technique is proposed and applied to
molecular dynamics simulations of the TIP4P water. The formula is analyzed and
optimal parameters of the Ewald method are identified. A comparison of the
obtained results with those evaluated within the reaction field approach is
made.Comment: 14 pages, 3 figure
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
Efficient algorithms for rigid body integration using optimized splitting methods and exact free rotational motion
Hamiltonian splitting methods are an established technique to derive stable
and accurate integration schemes in molecular dynamics, in which additional
accuracy can be gained using force gradients. For rigid bodies, a tradition
exists in the literature to further split up the kinetic part of the
Hamiltonian, which lowers the accuracy. The goal of this note is to comment on
the best combination of optimized splitting and gradient methods that avoids
splitting the kinetic energy. These schemes are generally applicable, but the
optimal scheme depends on the desired level of accuracy. For simulations of
liquid water it is found that the velocity Verlet scheme is only optimal for
crude simulations with accuracies larger than 1.5%, while surprisingly a
modified Verlet scheme (HOA) is optimal up to accuracies of 0.4% and a fourth
order gradient scheme (GIER4) is optimal for even higher accuracies.Comment: 2 pages, 1 figure. Added clarifying comments. Accepted for
publication in the Journal of Chemical Physic
Processed Splitting Algorithms for Rigid-Body Molecular Dynamics Simulations
A new approach for integration of motion in many-body systems of interacting
polyatomic molecules is proposed. It is based on splitting time propagation of
pseudo-variables in a modified phase space, while the real translational and
orientational coordinates are decoded by processing transformations. This
allows to overcome the barrier on the order of precision of the integration at
a given number of force-torque evaluations per time step. Testing in dynamics
of water versus previous methods shows that the obtained algorithms
significantly improve the accuracy of the simulations without extra
computational costs.Comment: 6 pages, 2 figures, submitted to Physical Review