c CCLRC
Section 2.3
The contribution to be added to the atomic stress tensor is given by
= -r
i
f
i
,
(2.115)
where and indicate the x, y, z components. The stress tensor is symmetric.
Interpolation arrays, vter and gter (set up in tersoff generate) - similar to those in van der
Waals interactions
, are used in the calculation of the Tersoff forces, virial and stress.
The Tersoff potentials are very short ranged, typically of order 3 °
A. This property, plus the fact
that Tersoff potentials (two- and three-body contributions) scale as N
3
, where N is the number of
particles, makes it essential that these terms are calculated by the link-cell method [
DL POLY 3 applies no long range corrections to the Tersoff potentials. In DL POLY 3 Tersoff
forces are handled by the routine tersoff forces.
2.3.4
Three Body Potentials
The three-body potentials in DL POLY 3 are mostly valence angle forms. (They are primarily
included to permit simulation of amorphous materials e.g. silicate glasses.) However, these have
been extended to include the Dreiding [
] hydrogen bond. The potential forms available are as
follows:
1. Harmonic: (harm)
U (
jik
) =
k
2
(
jik
-
0
)
2
(2.116)
2. Truncated harmonic: (thrm)
U (
jik
) =
k
2
(
jik
-
0
)
2
exp[-(r
8
ij
+ r
8
ik
)/
8
]
(2.117)
3. Screened Harmonic: (shrm)
U (
jik
) =
k
2
(
jik
-
0
)
2
exp[-(r
ij
/
1
+ r
ik
/
2
)]
(2.118)
U (
jik
) =
k
8(
jik
- )
2
(
0
- )
2
- (
jik
- )
2 2
×
exp[-(r
ij
/
1
+ r
ik
/
2
)]
(2.119)
U (
jik
) = k [
a
jik
(
jik
-
0
)
2
(
jik
+
0
- 2)
2
-
a
2
a-1
(
jik
-
0
)
2
( -
0
)
3
] exp[-(r
8
ij
+ r
8
ik
)/
8
]
(2.120)
6. Dreiding hydrogen bond [
U (
jik
) = D
hb
cos
4
(
jik
) [5(R
hb
/r
jk
)
12
- 6(R
hb
/r
jk
)
10
]
(2.121)
31