** Next:** Multiple Time Step Method
**Up:** Methods to Increase Efficiency
** Previous:** Methods to Increase Efficiency

##

Distance Cut-Off Scheme

The most commonly used approximation cuts-off all pair interactions beyond a
certain maximum distance
.
The cut-off is achieved by multiplying the force by a `cut-off'-function,
which depends only on the distance and
reduces the force to zero in a continous, differentiable,
manner. A typical `cut-off'-function assumes the constant value unity
up to a distance
, then decreases to zero
between
and
and remains zero for
all distances greater
than
. As a result, only interactions up to the
distance
have to be evaluated.
The appropriate choice of
is a compromise between
accuracy and computing time. On the one hand, the smaller
is chosen, the faster the computation will be
carried out.
On the other hand, a large
will reduce the error
of the approximation.

The `cut-off', although often employed, cannot be an appropriate
approximation for molecular dynamics simulations since a neglect of the Coulomb interaction is believed to
cause major rearrangements within the molecule.
An improved approximation scheme involves ordering of the long-range
interactions according to distance classes [44], as described
below. A similar algorithm had been suggested in [37] for
short-range forces in Lenard-Jones liquids.

** Next:** Multiple Time Step Method
**Up:** Methods to Increase Efficiency
** Previous:** Methods to Increase Efficiency
*Helmut Heller *

2000-04-19