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bond_style command

Syntax:

bond_style style args 
  args = none for any style except hybrid
  hybrid args = list of one or more styles 

Examples:

bond_style harmonic
bond_style fene
bond_style hybrid harmonic fene 

Description:

Set the formula(s) LAMMPS will use to compute bond interactions between pairs of atoms. The list of atom pairs is specified in the data or restart file and is read in by a read_data or read_restart command. The coefficients for the formula for each bond type can also be specified in those files or via the bond_coeff command. In all the formulas to follow, r is the distance between the 2 atoms in the bond.

A style of none means bond forces are not computed, even if bond are defined.


The class2 style uses the potential

where r0 is the equilibrium bond distance. Kn and r0 are coefficients defined for each bond type.


The fene style uses the potential

to define a finite extensible nonlinear elastic (FENE) potential (Kremer), used for bead-spring polymer models. The first term is attractive, the 2nd Lennard-Jones term is repulsive. The first term extends to R0, the maximum extent of the bond. The 2nd term is cutoff at 2^(1/6) sigma, the minimum of the LJ potential. K, R0, epsilon, and sigma are coefficients defined for each bond type.


The fene/expand style is similar to fene except that an extra shift factor of delta (positive or negative) is added to r to effectively change the bead size of the bonded atoms. The corresponding potential is

The first term now extends to R0 + delta and the 2nd term is cutoff at 2^(1/6) sigma + delta. K, R0, epsilon, sigma, and delta are coefficients defined for each bond type.


The harmonic style uses the potential

where r0 is the equilibrium bond distance. Note that the usual 1/2 factor is included in K. K and r0 are coefficients defined for each bond type.


The morse style uses the potential

where r0 is the equilibrium bond distance, alpha is a stiffness parameter, and D determines the depth of the potential well. D, alpha, and r0 are coefficients defined for each bond type.


The nonlinear style uses the potential

to define an anharmonic spring (Rector) of equilibrium length r0 and maximum extension lamda. Epsilon, r0, and lamda are coefficients defined for each bond type.


The hybrid style enables the use of multiple bond styles in one simulation. A bond style is assigned to each bond type. For example, bonds in a polymer flow (of bond type 1) could be computed with a fene potential and bonds in the wall boundary (of bond type 2) could be computed with a harmonic potential. The assignment of bond type to style is made via the bond_coeff command or in the data file.

Restrictions:

Bond styles can only be set for atom styles that allow bonds to be defined.

Bond styles are part of the "molecular" package. They are only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

Related commands:

bond_coeff, delete_bonds

Default:

bond_style none


(Kremer) Kremer, Grest, J Chem Phys, 92, 5057 (1990).

(Rector) Rector, Van Swol, Henderson, Molecular Physics, 82, 1009 (1994).