.. index:: fix set

fix set command
===============

Syntax
""""""

.. code-block:: LAMMPS

   fix ID group-ID set Nfreq set-args

* ID, group-ID are documented in :doc:`fix <fix>` command
* set = style name of this fix command
* Nfreq = reset per-atom properties every this many timesteps
* set-args = identical to args for the :doc:`set <set>` command
  
Examples
""""""""

.. code-block:: LAMMPS

   fix 10 all set 1 group all i_dump v_new
   fix 10 all set 1 group all i_dump v_turnoff

Description
"""""""""""

Reset one or more properties of one or more atoms once every *Nfreq*
stepes during a simulation.

The args following *Nfreq* are identical to those allowed for the
:doc:`set <set>` command, as in the examples above and below.

Note that the group-ID setting for this command is ignored.  The
syntax for the :doc:`set <set>` command allows selection of which
atoms have their properties reset.

This command can only be used to reset an atom property using a
per-atom variable.  This option in allowed by many, but not all, of
the keyword/value pairs supported by the :doc:`set <set>` command.
The reason for this restriction is that if a per-atom variable is not
used, this command will typically not change atom properties during
the simulation.

The :doc:`set <set>` command can be used with similar syntax to this
command to reset atom properties once before or between simulations.

----------

Here is an example of input script commands which will output atoms
into a dump file only when their x-velocity crosses a threshold value
*vthresh* for the first time.  Their position and x-velocity will then
be output every step for *twindow* timesteps.

.. code-block:: LAMMPS

   variable        vthresh equal 2             # threshold velocity
   variable        twindow equal 10            # dump for this many steps
   #
   # define custom property i_dump to store timestep threshold is crossed
   #
   fix             2 all property/atom i_dump
   set             group all i_dump -1
   #
   # fix set command checks for threshold crossings every step
   # resets i_dump from -1 to current timestep when crossing occurs
   #
   variable        start atom "vx > v_vthresh && i_dump == -1"
   variable        new atom ternary(v_start,step,i_dump)
   fix             3 all set 1 group all i_dump v_new
   #
   # dump command with thresh which enforces twindow
   #
   dump            1 all custom 1 tmp.dump id x y vx i_dump 
   variable        dumpflag atom "i_dump >= 0 && (step-i_dump) < v_twindow"
   dump_modify     1 thresh v_dumpflag == 1
   #
   # run the simulation
   # final dump with all atom IDs which crossed threshold on which timestep
   #
   run             1000
   write_dump      all custom tmp.dump.final id i_dump modify thresh i_dump >= 0

The tmp.dump.final file lists which atoms crossed the velocity
threshold.  This command will print the *twindow* timesteps when a
specific atom ID (104 in this case) was output in the tmp.dump file:

.. code-block:: LAMMPS

   % grep "^104 " tmp.dump
   
If these commands are used instead of the above, then an atom can
cross the velocity threshold multiple times, and will be output for
*twindow* timesteps each time.  However the write_dump command is no
longer useful.

.. code-block:: LAMMPS

   variable        vthresh equal 2             # threshold velocity
   variable        twindow equal 10            # dump for this many steps
   #
   # define custom property i_dump to store timestep threshold is crossed
   #
   fix             2 all property/atom i_dump
   set             group all i_dump -1
   #
   # fix set command checks for threshold crossings every step
   # resets i_dump from -1 to current timestep when crossing occurs
   #
   variable        start atom "vx > v_vthresh && i_dump == -1"
   variable        turnon atom ternary(v_start,step,i_dump)
   variable        stop atom "v_turnon >= 0 && (step-v_turnon) < v_twindow"
   variable        turnoff atom ternary(v_stop,v_turnon,-1)
   fix             3 all set 1 group all i_dump v_turnoff
   #
   # dump command with thresh which enforces twindow
   #
   dump            1 all custom 1 tmp.dump id x y vx i_dump 
   variable        dumpflag atom "i_dump >= 0 && (step-i_dump) < v_twindow"
   dump_modify     1 thresh v_dumpflag == 1
   #
   # run the simulation
   #
   run             1000
   
----------

Restart, fix_modify, output, run start/stop, minimize info
"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""

No information about this fix is written to :doc:`binary restart files
<restart>`.  None of the :doc:`fix_modify <fix_modify>` options are
relevant to this fix.  No global or per-atom quantities are stored by
this fix for access by various :doc:`output commands <Howto_output>`.
No parameter of this fix can be used with the *start/stop* keywords of
the :doc:`run <run>` command.  This fix is not invoked during
:doc:`energy minimization <minimize>`.

Restrictions
""""""""""""

As noted above, 


Related commands
""""""""""""""""

:doc:`set <set>`

Default
"""""""

none