c CCLRC
Section 2.4
Note that for the hydrogen bond, the hydrogen atom must be the central atom. Several of these
functions are identical to those appearing in the intra-molecular valence angle descriptions above.
There are significant differences in implementation however, arising from the fact that the three-
body potentials are regarded as inter-molecular. Firstly, the atoms involved are defined by atom
types, not specific indices. Secondly, there are no excluded atoms arising from the three-body
terms. (The inclusion of other potentials, for example pair potentials, may in fact be essential to
maintain the structure of the system.)
The three-body potentials are very short ranged, typically of order 3 °
A. This property, plus the
fact that three-body potentials scale as N
4
, where N is the number of particles, makes it essential
that these terms are calculated by the link-cell method [
The calculation of the forces, virial and stress tensor as described in the section valence angle
potentials above.
DL POLY 3 applies no long range corrections to the three-body potentials. The three-body forces
are calculated by the routine three body forces.
2.3.5
Four Body Potentials
The four-body potentials in DL POLY 3 are entirely inversion angle forms, primarily included
to permit simulation of amorphous materials (particularly borate glasses). The potential forms
available in DL POLY 3 are as follows:
1. Harmonic: (harm)
U (
ijkn
) =
k
2
(
ijkn
-
0
)
2
(2.122)
2. Harmonic cosine: (hcos)
U (
ijkn
) =
k
2
(cos(
ijkn
) - cos(
0
))
2
(2.123)
3. Planar potential: (plan)
U (
ijkn
) = A [1 - cos(
ijkn
)]
(2.124)
These functions are identical to those appearing in the intra-molecular inversion angle descriptions
above. There are significant differences in implementation however, arising from the fact that the
four-body potentials are regarded as inter-molecular. Firstly, the atoms involved are defined by
atom types, not specific indices. Secondly, there are no excluded atoms arising from the four-body
terms. (The inclusion of other potentials, for example pair potentials, may in fact be essential to
maintain the structure of the system.)
The four-body potentials are very short ranged, typically of order 3 °
A. This property, plus the fact
that four-body potentials scale as N
4
, where N is the number of particles, makes it essential that
these terms are calculated by the link-cell method [
The calculation of the forces, virial and stress tensor described in the section on inversion angle
potentials above.
DL POLY 3 applies no long range corrections to the four body potentials. The four-body forces
are calculated by the routine four body forces.
32