"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)
:line
pair_style dipole/cut command :h3
pair_style dipole/sf command :h3
[Syntax:]
pair_style dipole/cut cutoff (cutoff2) :pre
pair_style dipole/sf cutoff (cutoff2) :pre
cutoff = global cutoff LJ (and Coulombic if only 1 arg) (distance units)
cutoff2 = global cutoff for Coulombic (optional) (distance units) :ul
[Examples:]
pair_style dipole/cut 10.0
pair_coeff * * 1.0 1.0
pair_coeff 2 3 1.0 1.0 2.5 4.0 :pre
pair_style dipole/sf 9.0
pair_coeff * * 1.0 1.0
pair_coeff 2 3 1.0 1.0 2.5 4.0 :pre
[Description:]
Style {dipole/cut} computes interactions between pairs of particles
that each have a charge and/or a point dipole moment. In addition to
the usual Lennard-Jones interaction between the particles (Elj) the
charge-charge (Eqq), charge-dipole (Eqp), and dipole-dipole (Epp)
interactions are computed by these formulas for the energy (E), force
(F), and torque (T) between particles I and J.
:c,image(Eqs/pair_dipole.jpg)
where qi and qj are the charges on the two particles, pi and pj are
the dipole moment vectors of the two particles, r is their separation
distance, and the vector r = Ri - Rj is the separation vector between
the two particles. Note that Eqq and Fqq are simply Coulombic energy
and force, Fij = -Fji as symmetric forces, and Tij != -Tji since the
torques do not act symmetrically. These formulas are discussed in
"(Allen)"_#Allen and in "(Toukmaji)"_#Toukmaji.
Style {dipole/sf} computes 'shifted-force' interactions between pairs
of particles that each have a charge and/or a point dipole moment. In
general, a shifted-force potential is a (sligthly) modified potential
containing extra terms that make both the energy and its derivative go
to zero at the cutoff distance; this removes (cutoff-related) problems
in energy conservation and any numerical instability in the equations
of motion "(Allen)"_#Allen. Shifted-force interactions for the
Lennard-Jones (E_LJ), charge-charge (Eqq), charge-dipole (Eqp),
dipole-charge (Epq) and dipole-dipole (Epp) potentials are computed by
these formulas for the energy (E), force (F), and torque (T) between
particles I and J:
:c,image(Eqs/pair_dipole_sf.jpg)
where qi and qj are the charges on the two particles, pi and pj are
the dipole moment vectors of the two particles, r is their separation
distance, and the vector r = Ri - Rj is the separation vector between
the two particles. Note that Eqq and Fqq are simply Coulombic energy
and force, Fij = -Fji as symmetric forces, and Tij != -Tji since the
torques do not act symmetrically. The shifted-force formula for the
Lennard-Jones potential is reported in "(Stoddard)"_#Stoddard.
The original (unshifted) formulas for the electrostatic potentials,
forces and torques can be found in "(Price)"_#Price.
The shifted-force electrostatic potentials have been obtained by applying
equation 5.13 of "(Allen)"_#Allen. The formulas for the corresponding
forces and torques have been obtained by applying the
'chain rule' as in appendix C.3 of "(Allen)"_#Allen.
If one cutoff is specified in the pair_style command, it is used for
both the LJ and Coulombic (q,p) terms. If two cutoffs are specified,
they are used as cutoffs for the LJ and Coulombic (q,p) terms
respectively.
Atoms with dipole moments should be integrated using the "fix
nve/sphere update dipole"_fix_nve_sphere.html command to rotate the
dipole moments. The "compute temp/sphere"_compute_temp_sphere.html
command can be used to monitor the temperature, since it includes
rotational degrees of freedom. The "atom_style
dipole"_atom_style.html command should be used since it defines the
point dipoles and their rotational state. The magnitude of the dipole
moment for each type of particle can be defined by the
"dipole"_dipole.html command or in the "Dipoles" section of the data
file read in by the "read_data"_read_data.html command. Their initial
orientation can be defined by the "set dipole"_set.html command or in
the "Atoms" section of the data file.
The following coefficients must be defined for each pair of atoms
types via the "pair_coeff"_pair_coeff.html command as in the examples
above, or in the data file or restart files read by the
"read_data"_read_data.html or "read_restart"_read_restart.html
commands, or by mixing as described below:
epsilon (energy units)
sigma (distance units)
cutoff1 (distance units)
cutoff2 (distance units) :ul
The latter 2 coefficients are optional. If not specified, the global
LJ and Coulombic cutoffs specified in the pair_style command are used.
If only one cutoff is specified, it is used as the cutoff for both LJ
and Coulombic interactions for this type pair. If both coefficients
are specified, they are used as the LJ and Coulombic cutoffs for this
type pair.
:line
[Mixing, shift, table, tail correction, restart, rRESPA info]:
For atom type pairs I,J and I != J, the epsilon and sigma coefficients
and cutoff distances for this pair style can be mixed. The default
mix value is {geometric}. See the "pair_modify" command for details.
For atom type pairs I,J and I != J, the A, sigma, d1, and d2
coefficients and cutoff distance for this pair style can be mixed. A
is an energy value mixed like a LJ epsilon. D1 and d2 are distance
values and are mixed like sigma. The default mix value is
{geometric}. See the "pair_modify" command for details.
This pair style does not support the "pair_modify"_pair_modify.html
shift option for the energy of the Lennard-Jones portion of the pair
interaction; such energy goes to zero at the cutoff by construction.
The "pair_modify"_pair_modify.html table option is not relevant
for this pair style.
This pair style does not support the "pair_modify"_pair_modify.html
tail option for adding long-range tail corrections to energy and
pressure.
This pair style writes its information to "binary restart
files"_restart.html, so pair_style and pair_coeff commands do not need
to be specified in an input script that reads a restart file.
This pair style can only be used via the {pair} keyword of the
"run_style respa"_run_style.html command. It does not support the
{inner}, {middle}, {outer} keywords.
[Restrictions:]
This style is part of the "dipole" package. It is only enabled if
LAMMPS was built with that package. See the "Making
LAMMPS"_Section_start.html#2_3 section for more info.
[Related commands:]
"pair_coeff"_pair_coeff.html
[Default:] none
:line
:link(Allen)
[(Allen)] Allen and Tildesley, Computer Simulation of Liquids,
Clarendon Press, Oxford, 1987.
:link(Toukmaji)
[(Toukmaji)] Toukmaji, Sagui, Board, and Darden, J Chem Phys, 113,
10913 (2000).
:link(Stoddard)
[(Stoddard)] Stoddard and Ford, Phys Rev A, 8, 1504 (1973).
:link(Price)
[(Price)] Price, Stone and Alderton, Mol Phys, 52, 987 (1984).