c CCLRC
Section 2.4
2.4
Long Ranged Electrostatic (coulombic) Potentials
DL POLY 3 incorporates several techniques for dealing with long ranged electrostatic potentials
These are as follows:
1. Direct Coulomb sum
2. Force shifted Coulomb sum
3. Coulomb sum with distance dependent dielectric
4. Reaction field
5. Smoothed Particle Mesh Ewald (SPME)
All of these can be used in conjunction with the shell model technique used to account for ions
polarisation.
The SPME technique is restricted to periodic systems only. (Users must exercise care when using
pseudo-periodic boundary conditions.) The other techniques can be used with either periodic or
non-periodic systems safely, although in the case of the direct Coulomb sum there are likely to be
problems with convergence.
DL POLY 3 will correctly handle the electrostatics of both molecular and atomic species. However,
it is assumed that the system is electrically neutral. A warning message is printed if the system is
found to be charged, but otherwise the simulation proceeds as normal.
Note that DL POLY 3 does not use the basic Ewald method, which is an option in DL POLY 2,
on account of it being too slow for large scale systems. The SPME method is the standard Ewald
method in DL POLY 3 .
2.4.1
Direct Coulomb Sum
Use of the direct Coulomb sum is sometimes necessary for accurate simulation of isolated (non-
periodic) systems. It is not recommended for periodic systems.
The interaction potential for two charged ions is
U (r
ij
) =
1
4
0
q
i
q
j
r
ij
,
(2.125)
with q the charge on an atom labelled , and r
ij
the magnitude of the separation vector r
ij
= r
j
-r
i
.
The force on an atom j derived from this force is
f
j
=
1
4
0
q
i
q
j
r
3
ij
r
ij
,
(2.126)
with the force on atom i the negative of this.
The contribution to the atomic virial is
W = -
1
4
0
q
i
q
j
r
ij
,
(2.127)
2
Unlike the other elements of the force field, the electrostatic forces are NOT specified in the input FIELD file,
but by setting appropriate directives in the CONTROL file. See Section
33