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3.7. Bond, angle, dihedral, improper styles

Classes that compute molecular interactions are derived from the Bond, Angle, Dihedral, and Improper classes. New styles can be created to add new potentials to LAMMPS.

Writing new bond, angle, dihedral, and improper styles contains a detailed discussion of writing new bond, angle, dihedral, and improper styles from scratch with examples from the existing harmonic styles.

Bond_harmonic.cpp is the simplest example of a bond style. Ditto for the harmonic forms of the angle, dihedral, and improper style commands.

Here is a brief description of common methods you define in your new derived class. See bond.h, angle.h, dihedral.h, and improper.h for details and specific additional methods.

Required	“pure” methods that must be overridden in a derived class
compute	compute the molecular interactions for all listed items
coeff	set coefficients for one type
equilibrium_distance	length of bond, used by SHAKE (bond styles only)
equilibrium_angle	opening of angle, used by SHAKE (angle styles only)
write & read_restart	writes/reads coeffs to restart files
single	force/r (bond styles only) and energy of a single bond or angle

Optional	methods that have a default or dummy implementation
init	check if all coefficients are set, calls init_style()
init_style	check if style specific conditions are met
settings	apply global settings for all types
write & read_restart_settings	writes/reads global style settings to restart files
write_data	write corresponding Coeffs section(s) in data file
memory_usage	tally memory allocated by the style
extract	provide access to internal data
born_matrix	compute Born matrix elements for elasticity
reinit	reset all type-based parameters, called by fix adapt (bonds only)
pack & unpack_forward_comm	copy data to and from buffer in forward communication (bonds only)
pack & unpack_reverse_comm	copy data to and from buffer in reverse communication (bonds only)

Here is a list of flags or settings that should be set in the constructor of the derived class when they differ from the default setting.

Name of flag	Description	default
writedata	1 if write_data() is implemented	1
single_extra	number of extra single values calculated (bond styles only)	0
partial_flag	1 if bond type can be set to 0 and deleted (bond styles only)	0
reinitflag	1 if style has reinit() and is compatible with fix adapt	1
comm_forward	size of buffer (in doubles) for forward communication (bond styles only)	0
comm_reverse	size of buffer (in doubles) for reverse communication (bond styles only)	0
comm_reverse_off	size of buffer for reverse communication with newton off (bond styles only)	0
born_matrix_enable	1 if born_matrix() is implemented	0
centroidstressflag	CENTROID_AVAIL if centroid stress is different and implemented	varies

The default value of centroidstressflag is CENTROID_SAME for bond styles and CENTROID_AVAIL for angle, dihedral, and improper styles. It should be set to CENTROID_NOTAVAIL when the centroid stress differs from the two-body virial but has not yet been implemented.