fix python command


fix ID group-ID python N callback function_name
  • ID, group-ID are ignored by this fix

  • python = style name of this fix command

  • N = execute every N steps

  • callback = post_force or end_of_step

    post_force = callback after force computations on atoms every N time steps
    end_of_step = callback after every N time steps


python post_force_callback here """
from lammps import lammps

def post_force_callback(lammps_ptr, vflag):
    lmp = lammps(ptr=lammps_ptr)
    # access LAMMPS state using Python interface

python end_of_step_callback here """
def end_of_step_callback(lammps_ptr):
    lmp = lammps(ptr=lammps_ptr)
    # access LAMMPS state using Python interface

fix pf  all python 50 post_force post_force_callback
fix eos all python 50 end_of_step end_of_step_callback


This fix allows you to call a Python function during a simulation run. The callback is either executed after forces have been applied to atoms or at the end of every N time steps.

Callback functions must be declared in the global scope of the active Python interpreter. This can either be done by defining it inline using the python command or by importing functions from other Python modules. If LAMMPS is driven using the library interface from Python, functions defined in the driving Python interpreter can also be executed.

Each callback is given a pointer object as first argument. This can be used to initialize an instance of the lammps Python interface, which gives access to the LAMMPS state from Python.


While you can access the state of LAMMPS via library functions from these callbacks, trying to execute input script commands will in the best case not work or in the worst case result in undefined behavior.


This fix is part of the PYTHON package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

Building LAMMPS with the PYTHON package will link LAMMPS with the Python library on your system. Settings to enable this are in the lib/python/Makefile.lammps file. See the lib/python/README file for information on those settings.