"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)

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pair_style coul/cut command :h3
pair_style coul/cut/omp command :h3
pair_style coul/debye command :h3
pair_style coul/debye/omp command :h3
pair_style coul/dsf command :h3
pair_style coul/dsf/gpu command :h3
pair_style coul/dsf/omp command :h3
pair_style coul/long command :h3
pair_style coul/long/omp command :h3
pair_style coul/long/gpu command :h3
pair_style coul/msm command :h3
pair_style coul/msm/omp command :h3
pair_style coul/wolf command :h3
pair_style coul/wolf/omp command :h3
pair_style tip4p/cut command :h3
pair_style tip4p/long command :h3
pair_style tip4p/cut/omp command :h3
pair_style tip4p/long/omp command :h3

[Syntax:]

pair_style coul/cut cutoff
pair_style coul/debye kappa cutoff
pair_style coul/dsf alpha cutoff
pair_style coul/long cutoff
pair_style coul/long/gpu cutoff 
pair_style coul/wolf alpha cutoff
pair_style tip4p/cut otype htype btype atype qdist cutoff
pair_style tip4p/long otype htype btype atype qdist cutoff :pre

cutoff = global cutoff for Coulombic interactions
kappa = Debye length (inverse distance units) 
alpha = damping parameter (inverse distance units) :ul

[Examples:]

pair_style coul/cut 2.5
pair_coeff * *
pair_coeff 2 2 3.5 :pre

pair_style coul/debye 1.4 3.0
pair_coeff * *
pair_coeff 2 2 3.5 :pre

pair_style coul/dsf 0.05 10.0
pair_coeff * * :pre

pair_style coul/long 10.0
pair_coeff * * :pre

pair_style coul/msm 10.0
pair_coeff * * :pre

pair_style coul/wolf 0.2 9.0
pair_coeff * * :pre

pair_style tip4p/cut 1 2 7 8 0.15 12.0
pair_coeff * * :pre

pair_style tip4p/long 1 2 7 8 0.15 10.0
pair_coeff * * :pre

[Description:]

The {coul/cut} style computes the standard Coulombic interaction
potential given by

:c,image(Eqs/pair_coulomb.jpg)

where C is an energy-conversion constant, Qi and Qj are the charges on
the 2 atoms, and epsilon is the dielectric constant which can be set
by the "dielectric"_dielectric.html command.  The cutoff Rc truncates
the interaction distance.

Style {coul/debye} adds an additional exp() damping factor to the
Coulombic term, given by

:c,image(Eqs/pair_debye.jpg)

where kappa is the Debye length.  This potential is another way to
mimic the screening effect of a polar solvent.

Style {coul/dsf} computes Coulombic interactions via the damped 
shifted force model described in "Fennell"_#Fennell, given by:

:c,image(Eqs/pair_coul_dsf.jpg)

where {alpha} is the damping parameter and erfc() is the
complementary error-function. The potential corrects issues in the
Wolf model (described below) to provide consistent forces and energies
(the Wolf potential is not differentiable at the cutoff) and smooth
decay to zero.

Style {coul/wolf} computes Coulombic interactions via the Wolf
summation method, described in "Wolf"_#Wolf, given by:

:c,image(Eqs/pair_coul_wolf.jpg)

where {alpha} is the damping parameter, and erc() and erfc() are
error-fuction and complementary error-function terms.  This potential
is essentially a short-range, spherically-truncated,
charge-neutralized, shifted, pairwise {1/r} summation.  With a
manipulation of adding and substracting a self term (for i = j) to the
first and second term on the right-hand-side, respectively, and a
small enough {alpha} damping parameter, the second term shrinks and
the potential becomes a rapidly-converging real-space summation.  With
a long enough cutoff and small enough alpha parameter, the energy and
forces calcluated by the Wolf summation method approach those of the
Ewald sum.  So it is a means of getting effective long-range
interactions with a short-range potential.

Styles {coul/long} and {coul/msm} compute the same Coulombic
interactions as style {coul/cut} except that an additional damping
factor is applied so it can be used in conjunction with the
"kspace_style"_kspace_style.html command and its {ewald} or {pppm}
option.  The Coulombic cutoff specified for this style means that
pairwise interactions within this distance are computed directly;
interactions outside that distance are computed in reciprocal space.

Styles {tip4p/cut} and {tip4p/long} implement the coulomb part of
the TIP4P water model of "(Jorgensen)"_#Jorgensen, which introduces
a massless site located a short distance away from the oxygen atom
along the bisector of the HOH angle.  The atomic types of the oxygen and
hydrogen atoms, the bond and angle types for OH and HOH interactions,
and the distance to the massless charge site are specified as
pair_style arguments.  Style {tip4p/cut} uses a global cutoff for
Coulomb interactions; style {tip4p/long} is for use with a long-range
Coulombic solver (Ewald or PPPM).

IMPORTANT NOTE: For each TIP4P water molecule in your system, the atom
IDs for the O and 2 H atoms must be consecutive, with the O atom
first.  This is to enable LAMMPS to "find" the 2 H atoms associated
with each O atom.  For example, if the atom ID of an O atom in a TIP4P
water molecule is 500, then its 2 H atoms must have IDs 501 and 502.

See the "howto section"_Section_howto.html#howto_8 for more
information on how to use the TIP4P pair styles and lists of
parameters to set.  Note that the neighobr list cutoff for Coulomb
interactions is effectively extended by a distance 2*qdist when using
the TIP4P pair style, to account for the offset distance of the
fictitious charges on O atoms in water molecules.  Thus it is
typically best in an efficiency sense to use a LJ cutoff >= Coulomb
cutoff + 2*qdist, to shrink the size of the neighbor list.  This leads
to slightly larger cost for the long-range calculation, so you can
test the trade-off for your model.

These potentials are designed to be combined with other pair
potentials via the "pair_style hybrid/overlay"_pair_hybrid.html
command.  This is because they have no repulsive core.  Hence if they
are used by themselves, there will be no repulsion to keep two
oppositely charged particles from overlapping each other.

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:

cutoff (distance units) :ul

For {coul/cut} and {coul/debye}, the cutoff coefficient is optional.
If it is not used (as in some of the examples above), the default
global value specified in the pair_style command is used.

For {coul/long} and {coul/msm} no cutoff can be specified for an
individual I,J type pair via the pair_coeff command.  All type pairs
use the same global Coulombic cutoff specified in the pair_style
command.

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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.

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[Mixing, shift, table, tail correction, restart, rRESPA info]:

For atom type pairs I,J and I != J, the cutoff distance for the
{coul/cut} style can be mixed.  The default mix value is {geometric}.
See the "pair_modify" command for details.

The "pair_modify"_pair_modify.html shift option is not relevant
for these pair styles.

The {coul/long} style supports the "pair_modify"_pair_modify.html
table option for tabulation of the short-range portion of the
long-range Coulombic interaction.

These pair styles do not support the "pair_modify"_pair_modify.html
tail option for adding long-range tail corrections to energy and
pressure.

These pair styles write their 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.

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[Restrictions:]

The {coul/long}, {coul/msm} and {tip4p/long} styles are part of the
KSPACE package.  They are only enabled if LAMMPS was built with that
package (which it is by default).
 See the "Making LAMMPS"_Section_start.html#start_3 section
for more info.

[Related commands:]

"pair_coeff"_pair_coeff.html, "pair_style
hybrid/overlay"_pair_hybrid.html
"kspace_style"_kspace_style.html

[Default:] none

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:link(Wolf)
[(Wolf)] D. Wolf, P. Keblinski, S. R. Phillpot, J. Eggebrecht, J Chem
Phys, 110, 8254 (1999).

:link(Fennell)
[(Fennell)] C. J. Fennell, J. D. Gezelter, J Chem Phys, 124, 
234104 (2006).
