"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/debye command :h3
pair_style coul/long command :h3

[Syntax:]

pair_style coul/cut cutoff
pair_style coul/debye kappa cutoff
pair_style coul/long cutoff :pre

cutoff = global cutoff for Coulombic interactions
kappa = Debye length (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/long 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/long} computes 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.

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} 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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[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} style is part of the "kspace" package.  It is only
enabled if LAMMPS was built with that package (which it is by
default).  See the "Making LAMMPS"_Section_start.html#2_3 section for
more info.

On some 64-bit machines, compiling with -O3 appears to break the
Coulombic tabling option used by the {coul/long} style.  See the
"Additional build tips" section of the Making LAMMPS documentation
pages for workarounds on this issue.

[Related commands:]

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

[Default:] none
