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### pair_style cg/cmm command

### pair_style cg/cmm/gpu command

### pair_style cg/cmm/coul/cut command

### pair_style cg/cmm/coul/long command

### pair_style cg/cmm/coul/long/gpu command

Syntax:

    pair_style style args

- style = cg/cmm or cg/cmm/gpu or cg/cmm/coul/cut or cg/cmm/coul/long or cg/cmm/coul/long/gpu
- args = list of arguments for a particular style

      cg/cmm args = cutoff
        cutoff = global cutoff for Lennard Jones interactions (distance units)
      cg/cmm/gpu 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)

      cg/cmm/coul/long/gpu args = cutoff (cutoff2)
        cutoff = global cutoff for LJ (and Coulombic if only 1 arg) (distance units)
        cutoff2 = global cutoff for Coulombic (optional) (distance units)

Examples:

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

    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

    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

Description:

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

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

Style cg/cmm/gpu is a GPU-enabled version of style cg/cmm. See more details below.

Style cg/cmm/coul/cut adds a Coulombic pairwise interaction given by

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 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

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 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.

Style cg/cmm/coul/long/gpu is a GPU-enabled version of style cg/cmm/coul/long. See more details below.

The following coefficients must be defined for each pair of atoms types via the pair_coeff command as in the examples above, or in the data file or restart files read by the read_data or read_restart 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)

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.

The cg/cmm/gpu and cg/cmm/coul/long/gpu styles are identical to the cg/cmm and cg/cmm/coul/long styles, except that each processor off-loads its pairwise calculations to a GPU chip. Depending on the hardware available on your system this can provide a speed-up. See the Running on GPUs section of the manual for more details about hardware and software requirements for using GPUs.

More details about these settings and various possible hardware configuration are in this section of the manual.

Additional requirements in your input script to run with GPU-enabled styles are as follows:

The newton pair setting must be off and fix gpu must be used. The fix controls the essential GPU selection and initialization steps.

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 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 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, 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 command, meaning the pairwise forces can be partitioned by distance at different levels of the rRESPA hierarchy. See the run_style command for details.

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 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, angle_style cg/cmm

Default: none

(Shinoda) Shinoda, DeVane, Klein, Mol Sim, 33, 27 (2007).

(DeVane) Shinoda, DeVane, Klein, Soft Matter, 4, 2453-2462 (2008).