# 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.

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 &                 writes/reads global style settings to restart
  read_restart_settings   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 (bond or angle
                          styles only)

  reinit                  reset all type-based parameters, called by fix
                          adapt (bonds only)

  pack &                  copy data to and from buffer in forward
  unpack_forward_comm     communication (bonds only)

  pack &                  copy data to and from buffer in reverse
  unpack_reverse_comm     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      0
                      (bond styles only)                            

  partial_flag        1 if bond type can be set to 0 and deleted    0
                      (bond styles only)                            

  reinitflag          1 if style has reinit() and is compatible     1
                      with fix adapt                                

  comm_forward        size of buffer (in doubles) for forward       0
                      communication (bond styles only)              

  comm_reverse        size of buffer (in doubles) for reverse       0
                      communication (bond styles only)              

  comm_reverse_off    size of buffer for reverse communication with 0
                      newton off (bond styles only)                 
  ---------------------------------------------------------------------------