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### bond_style quartic command

Syntax:

    bond_style quartic

Examples:

    bond_style quartic
    bond_coeff 2 1200 -0.55 0.25 1.3 34.6878

Description:

The quartic bond style uses the potential

to define a bond that can be broken as the simulation proceeds (e.g. due to a polymer being stretched). The sigma and epsilon used in the LJ portion of the formula are both set equal to 1.0 by LAMMPS.

The following coefficients must be defined for each bond type via the bond_coeff command as in the example above, or in the data file or restart files read by the read_data or read_restart commands:

- K (energy/distance^2)
- B1 (distance)
- B2 (distance)
- Rc (distance)
- U0 (energy)

This potential was constructed to mimic the FENE bond potential for coarse-grained polymer chains. When monomers with sigma = epsilon = 1.0 are used, the following choice of parameters gives a quartic potential that looks nearly like the FENE potential: K = 1200, B1 = -0.55, B2 = 0.25, Rc = 1.3, and U0 = 34.6878. Different parameters can be specified using the bond_coeff command, but you will need to choose them carefully so they form a suitable bond potential.

Rc is the cutoff length at which the bond potential goes smoothly to a local maximum. If a bond length ever becomes > Rc, LAMMPS "breaks" the bond, which means two things. First, the bond potential is turned off by setting its type to 0, and is no longer computed. Second, a pairwise interaction between the two atoms is turned on, since they are no longer bonded.

LAMMPS does the second task via a computational sleight-of-hand. It subtracts the pairwise interaction as part of the bond computation. When the bond breaks, the subtraction stops. For this to work, the pairwise interaction must always be computed by the pair_style command, whether the bond is broken or not. This means that special_bonds must be set to 1,1,1, as indicated as a restriction below.

Note that when bonds are dumped to a file via dump bond, bonds with type 0 are not included. The delete_bonds command can also be used to query the status of broken bonds or permanently delete them, e.g.:

    delete_bonds all stats
    delete_bonds all bond 0 remove

Restrictions:

This bond style can only be used if LAMMPS was built with the "molecular" package (which it is by default). See the Making LAMMPS section for more info on packages.

The quartic style requires that special_bonds parameters be set to 1,1,1. Three- and four-body interactions (angle, dihedral, etc) cannot be used with quartic bonds.

Related commands:

bond_coeff, delete_bonds

Default: none