lammps/doc/pair_dipole.txt

"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 lj/cut/dipole/cut command :h3
pair_style lj/cut/dipole/cut/gpu command :h3
pair_style lj/cut/dipole/cut/omp command :h3
pair_style lj/sf/dipole/sf command :h3
pair_style lj/sf/dipole/sf/gpu command :h3
pair_style lj/sf/dipole/sf/omp command :h3
pair_style lj/cut/dipole/long command :h3
pair_style lj/long/dipole/long command :h3

[Syntax:]

pair_style lj/cut/dipole/cut cutoff (cutoff2)
pair_style lj/sf/dipole/sf cutoff (cutoff2)
pair_style lj/cut/dipole/long cutoff (cutoff2)
pair_style lj/long/dipole/long flag_lj flag_coul cutoff (cutoff2) :pre

cutoff = global cutoff LJ (and Coulombic if only 1 arg) (distance units) :ulb,l
cutoff2 = global cutoff for Coulombic and dipole (optional) (distance units) :l
flag_lj = {long} or {cut} or {off} :l
  {long} = use long-range damping on dispersion 1/r^6 term
  {cut} = use a cutoff on dispersion 1/r^6 term
  {off} = omit disperion 1/r^6 term entirely :pre
flag_coul = {long} or {off} :l
  {long} = use long-range damping on Coulombic 1/r and point-dipole terms
  {off} = omit Coulombic and point-dipole terms entirely :pre
:ule

[Examples:]

pair_style lj/cut/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 lj/sf/dipole/sf 9.0
pair_coeff * * 1.0 1.0 
pair_coeff 2 3 1.0 1.0 2.5 4.0 :pre

pair_style lj/cut/dipole/long 10.0
pair_coeff * * 1.0 1.0 
pair_coeff 2 3 1.0 1.0 2.5 4.0 :pre

pair_style lj/long/dipole/long long long 3.5 10.0
pair_coeff * * 1.0 1.0 
pair_coeff 2 3 1.0 1.0 2.5 4.0 :pre

[Description:]

Style {lj/cut/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 {lj/sf/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)
:c,image(Eqs/pair_dipole_sf2.jpg)

where epsilon and sigma are the standard LJ parameters, r_c is the
cutoff, 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.

Style {lj/cut/dipole/long} computes long-range point-dipole
interactions as discussed in "(Toukmaji)"_#Toukmaji. Dipole-dipole,
dipole-charge, and charge-charge interactions are all supported, along
with the standard 12/6 Lennard-Jones interactions, which are computed
with a cutoff.  A "kspace_style"_kspace_style.html must be defined to
use this pair style.  Currently, only "kspace_style
ewald/disp"_kspace_style.html support long-range point-dipole
interactions.

Style {lj/long/dipole/long} also computes point-dipole interactions as
discussed in "(Toukmaji)"_#Toukmaji. Long-range dipole-dipole,
dipole-charge, and charge-charge interactions are all supported, along
with the standard 12/6 Lennard-Jones interactions.  LJ interactions
can be cutoff or long-ranged.

For style {lj/long/dipole/long}, if {flag_lj} is set to {long}, no
cutoff is used on the LJ 1/r^6 dispersion term.  The long-range
portion is calculated by using the "kspace_style
ewald_disp"_kspace_style.html command.  The specified LJ cutoff then
determines which portion of the LJ interactions are computed directly
by the pair potential versus which part is computed in reciprocal
space via the Kspace style.  If {flag_lj} is set to {cut}, the LJ
interactions are simply cutoff, as with "pair_style
lj/cut"_pair_lj.html.  If {flag_lj} is set to {off}, LJ interactions
are not computed at all.

If {flag_coul} is set to {long}, no cutoff is used on the Coulombic or
dipole interactions.  The long-range portion is calculated by using
{ewald_disp} of the "kspace_style"_kspace_style.html command. If
{flag_coul} is set to {off}, Coulombic and dipole interactions are not
computed at all.

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 {omega} option on the "fix
langevin"_fix_langevin.html command can be used to thermostat the
rotational motion.  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

Styles with a {cuda}, {gpu}, {omp}, or {opt} suffix are functionally
the same as the corresponding style without the suffix.  They have
been optimized to run faster, depending on your available hardware, as
discussed in "Section_accelerate"_Section_accelerate.html of the
manual.  The accelerated styles take the same arguments and should
produce the same results, except for round-off and precision issues.

These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
packages, respectively.  They are only enabled if LAMMPS was built with
those packages.  See the "Making LAMMPS"_Section_start.html#start_3
section for more info.

You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the "-suffix command-line
switch"_Section_start.html#start_7 when you invoke LAMMPS, or you can
use the "suffix"_suffix.html command in your input script.

See "Section_accelerate"_Section_accelerate.html of the manual for
more instructions on how to use the accelerated styles effectively.

: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:]

The {lj/cut/dipole/cut}, {lj/cut/dipole/long}, and
{lj/long/dipole/long} styles are part of the DIPOLE package.  They are
only enabled if LAMMPS was built with that package.  See the "Making
LAMMPS"_Section_start.html#start_3 section for more info.

The {lj/sf/dipole/sf} style is part of the USER-MISC package.  It is
only enabled if LAMMPS was built with that package.  See the "Making
LAMMPS"_Section_start.html#start_3 section for more info.

Using dipole pair styles with {electron} "units"_units.html is not
currently supported.

[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).