c CCLRC
Section 5.1
Table 5.16: Four-body Potentials
key
potential type
Variables (1-2)
functional form
harm
Harmonic
k
0
U () =
k
2
( -
0
)
2
hcos
Harmonic cosine
k
0
U () =
k
2
(cos() - cos(
0
))
2
plan
Planar
A
U () = A [1 - cos()]
is the i-j-k-l inversion angle
Note that the third variable is the range at which the four-body potential is truncated. The
distance is in °
A, measured from the central atom.
5.1.3.3
External Field
The presence of an external field is flagged by the directive:
extern
The following line in the FIELD file must have another directive indicating what type of field
is to be applied. The variables pertaining to each potential are described in Table
Note: only one type of field can be applied at a time.
Table 5.17: External Fields
key
potential type
Variables (1-4)
functional form
elec
Electric Field
E
x
E
y
E
z
F = q E
oshm
Oscillating Shear
A
n
F
x
= A cos(2n z/L
z
)
shrx
Continuous Shear
A
z
0
v
x
=
A
2
|z|
z
: |z| > z
0
grav
Gravitational Field
G
x
G
y
G
z
F = m G
magn
Magnetic Field
H
x
H
y
H
z
F = q (v × H)
sphr
Containing Sphere
A
R
0
n
R
cut
F = A (R
0
- r)
-n
: r > R
cut
zbnd
Repulsive Wall
A
z
0
p
F = A (z
0
- z) : p z > p z
0
100