.. index:: fix_modify

fix_modify command
==================

Syntax
""""""

.. parsed-literal::

   fix_modify fix-ID keyword value ...

* fix-ID = ID of the fix to modify
* one or more keyword/value pairs may be appended
* keyword = *temp* or *press* or *energy* or *respa*
.. parsed-literal::

     *temp* value = compute ID that calculates a temperature
     *press* value = compute ID that calculates a pressure
     *energy* value = *yes* or *no*
     *respa* value = *1* to *max respa level* or *0* (for outermost level)



Examples
""""""""

.. parsed-literal::

   fix_modify 3 temp myTemp press myPress
   fix_modify 1 energy yes
   fix_modify tether respa 2

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

Modify one or more parameters of a previously defined fix.  Only
specific fix styles support specific parameters.  See the doc pages
for individual fix commands for info on which ones support which
fix_modify parameters.

The *temp* keyword is used to determine how a fix computes
temperature.  The specified compute ID must have been previously
defined by the user via the :doc:`compute <compute>` command and it must
be a style of compute that calculates a temperature.  All fixes that
compute temperatures define their own compute by default, as described
in their documentation.  Thus this option allows the user to override
the default method for computing T.

The *press* keyword is used to determine how a fix computes pressure.
The specified compute ID must have been previously defined by the user
via the :doc:`compute <compute>` command and it must be a style of
compute that calculates a pressure.  All fixes that compute pressures
define their own compute by default, as described in their
documentation.  Thus this option allows the user to override the
default method for computing P.

For fixes that calculate a contribution to the potential energy of the
system, the *energy* keyword will include that contribution in
thermodynamic output of potential energy.  This is because the *energy
yes* setting must be specfied to include the fix's global or per-atom
energy in the calculation performed by the :doc:`compute pe <compute_pe>` or :doc:`compute pe/atom <compute_pe_atom>`
commands.  See the :doc:`thermo_style <thermo_style>` command for info
on how potential energy is output.  For fixes that tally a global
energy, it can be printed by using the keyword f_ID in the
thermo_style custom command, where ID is the fix-ID of the appropriate
fix.

.. note::

   You must also specify the *energy yes* setting for a fix if you
   are using it when performing an :doc:`energy minimization <minimize>`
   and if you want the energy and forces it produces to be part of the
   optimization criteria.

For fixes that set or modify forces, it may be possible to select at
which :doc:`r-RESPA <run_style>` level the fix operates via the *respa*
keyword. The RESPA level at which the fix is active can be selected.
This is a number ranging from 1 to the number of levels. If the RESPA
level is larger than the current maximum, the outermost level will be
used, which is also the default setting. This default can be restored
using a value of *0* for the RESPA level. The affected fix has to be
enabled to support this feature; if not, *fix_modify* will report an
error. Active fixes with a custom RESPA level setting are reported
with their specified level at the beginning of a r-RESPA run.

Restrictions
""""""""""""
 none

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

:doc:`fix <fix>`, :doc:`compute temp <compute_temp>`, :doc:`compute pressure <compute_pressure>`, :doc:`thermo_style <thermo_style>`

Default
"""""""

The option defaults are temp = ID defined by fix, press = ID defined
by fix, energy = no, respa = 0.


.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm