c CCLRC
Section 2.1
2.1
Introduction to DL POLY 3 Force Field
The force field is the set of functions needed to define the interactions in a molecular system. These
may have a wide variety of analytical forms, with some basis in chemical physics, which must be
parameterised to give the correct energy and forces. A huge variety of forms is possible and for this
reason the DL POLY 3 force field is designed to be adaptable. While it is not supplied with its own
force field parameters, many of the functions familiar to GROMOS [
] users have been coded in the package, as well as less familiar forms. In addition DL POLY 3
retains the possibility of the user defining additional potentials.
In DL POLY 3 the total configuration energy of a molecular system may be written as:
U (r
1
, r
2
, . . . , r
N
) =
N
shel
i
shel
=1
U
shel
(i
shel
, r
core
, r
shell
)
+
N
teth
i
teth
=1
U
teth
(i
teth
, r
i
( = t), r
i
( = 0)
+
N
bond
i
bond
=1
U
bond
(i
bond
, r
a
, r
b
)
+
N
angl
i
angl
=1
U
angl
(i
angl
, r
a
, r
b
, r
c
)
+
N
dihd
i
dihd
=1
U
dihd
(i
dihd
, r
a
, r
b
, r
c
, r
d
)
+
N
inv
i
inv
=1
U
inv
(i
inv
, r
a
, r
b
, r
c
, r
d
)
+
N -1
i=1
N
j>i
U
(metal)
2 body
(i, j, |r
i
- r
j
|)
(2.1)
+
N
i=1
N
j=i
N
k=j
U
tersof f
(i, j, k, r
i
, r
j
, r
k
)
+
N -2
i=1
N -1
j>i
N
k>j
U
3 body
(i, j, k, r
i
, r
j
, r
k
)
+
N -3
i=1
N -2
j>i
N -1
k>j
N
n>k
U
4 body
(i, j, k, n, r
i
, r
j
, r
k
, r
n
)
+
N
i=1
U
extn
(i, r
i
, v
i
) ,
where U
shel
, U
teth
, U
bond
, U
angl
, U
dihd
, U
inv
, U
pair
, U
tersof f
, U
3 body
and U
4 body
are empirical
interaction functions representing ion core-shell polarisation, tethered particles, chemical bonds,
valence angles, dihedral (and improper dihedral angles), inversion angles, two-body, Tersoff, three-
body and four-body forces respectively. The first six are regarded by DL POLY 3 as intra-molecular
interactions and the next four as inter-molecular interactions. The final term U
extn
represents an
external field potential. The position vectors r
a
, r
b
, r
c
and r
d
refer to the positions of the atoms
specifically involved in a given interaction. (Almost universally, it is the differences in position
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