"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 cg/cmm command :h3
pair_style cg/cmm/coul/cut command :h3
pair_style cg/cmm/coul/long command :h3

[Syntax:]

pair_style style args :pre

style = {cg/cmm} or {cg/cmm/coul/cut} or {cg/cmm/coul/long}
args = list of arguments for a particular style :ul
  {cg/cmm} args = cutoff
    cutoff = global cutoff for Lennard Jones interactions (distance units)
  {cg/cmm/coul/cut} args = cutoff (cutoff2) (kappa)
    cutoff = global cutoff for LJ (and Coulombic if only 1 arg) (distance units)
    cutoff2 = global cutoff for Coulombic (optional) (distance units)
    kappa = Debye length (optional, defaults to 0.0 = disabled) (inverse distance units)
  {cg/cmm/coul/long} args = cutoff (cutoff2)
    cutoff = global cutoff for LJ (and Coulombic if only 1 arg) (distance units)
    cutoff2 = global cutoff for Coulombic (optional) (distance units) :pre

[Examples:]

pair_style cg/cmm 2.5
pair_coeff 1 1 lj12_6 1 1.1 2.8 :pre

pair_style cg/cmm/coul/cut 10.0 12.0
pair_coeff 1 1 lj9_6  100.0 3.5 9.0
pair_coeff 1 1 lj12_4 100.0 3.5 9.0 9.0 :pre

pair_style cg/cmm/coul/long 10.0
pair_style cg/cmm/coul/long 10.0 8.0
pair_coeff 1 1 lj9_6 100.0 3.5 9.0 :pre

[Description:]

The {cg/cmm} styles compute a 9/6, 12/4, or 12/6 Lennard-Jones potential,
given by

:c,image(Eqs/pair_cmm.jpg)

as required for the CMM Coarse-grained MD parametrization discussed in
"(Shinoda)"_#Shinoda and "(DeVane)"_#DeVane.  Rc is the cutoff.

Style {cg/cmm/coul/cut} adds a Coulombic pairwise interaction 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.  If one cutoff is
specified in the pair_style command, it is used for both the LJ and
Coulombic terms.  If two cutoffs are specified, they are used as
cutoffs for the LJ and Coulombic terms respectively.

This style also contains an additional exp() damping factor
to the Coulombic term, given by

:c,image(Eqs/pair_debye.jpg)

where kappa is the Debye length (kappa=0.0 is the unscreened coulomb).
This potential is another way to mimic the screening effect of a polar
solvent.

Style {cg/cmm/coul/long} computes the same Coulombic interactions as
style {cg/cmm/coul/cut} except that an additional damping factor is
applied to the Coulombic term 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.

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:

cg_type (lj9_6, lj12_4, or lj12_6)
epsilon (energy units)
sigma (distance units)
cutoff1 (distance units)
cutoff2 (distance units) :ul

Note that sigma is defined in the LJ formula as the zero-crossing
distance for the potential, not as the energy minimum. The prefactors
are chosen so that the potential minimum is at -epsilon.

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.

For {cg/cmm/coul/long} only the LJ cutoff can be specified since a
Coulombic cutoff cannot be specified for an individual I,J type pair.
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, and rRESPA info]:

For atom type pairs I,J and I != J, the epsilon and sigma coefficients
and cutoff distance for all of the cg/cmm pair styles {cannot} be mixed,
since different pairs may have different exponents. So all parameters
for all pairs have to be specified explicitly through the "pair_coeff"
command. Defining then in a data file is also not supported, due to
limitations of that file format.

All of the cg/cmm pair styles support the
"pair_modify"_pair_modify.html shift option for the energy of the
Lennard-Jones portion of the pair interaction.

The {cg/cmm/coul/long} pair styles support the
"pair_modify"_pair_modify.html table option since they can tabulate
the short-range portion of the long-range Coulombic interaction.

All of the cg/cmm 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.

The cg/cmm, cg/cmm/coul/cut and lj/cut/coul/long pair styles support 
the use of the {inner}, {middle}, and {outer} keywords of the "run_style
respa"_run_style.html command, meaning the pairwise forces can be
partitioned by distance at different levels of the rRESPA hierarchy.
See the "run_style"_run_style.html command for details.

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[Restrictions:]

All of the cg/cmm pair styles are part of the "user-cg-cmm"
package. They are only enabled if LAMMPS was built with that
package. The {cg/cmm/coul/long} style also requires the "kspace"
package to be built (which is enabled 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 {cg/cmm/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, "angle_style cg/cmm"_angle_cmm.html

[Default:] none

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:link(Shinoda)
[(Shinoda)] Shinoda, DeVane, Klein, Mol Sim, 33, 27 (2007).

:link(DeVane)
[(DeVane)]  Shinoda, DeVane, Klein, Soft Matter, 4, 2453-2462 (2008).

