.. index:: fix adapt

fix adapt command
=================

Syntax
""""""


.. parsed-literal::

   fix ID group-ID adapt N attribute args ... keyword value ...

* ID, group-ID are documented in :doc:`fix <fix>` command
* adapt = style name of this fix command
* N = adapt simulation settings every this many timesteps
* one or more attribute/arg pairs may be appended
* attribute = *pair* or *kspace* or *atom*
  
  .. parsed-literal::
  
       *pair* args = pstyle pparam I J v_name
         pstyle = pair style name, e.g. lj/cut
         pparam = parameter to adapt over time
         I,J = type pair(s) to set parameter for
         v_name = variable with name that calculates value of pparam
       *bond* args = bstyle bparam I v_name
         bstyle = bond style name, e.g. harmonic
         bparam = parameter to adapt over time
         I = type bond to set parameter for
         v_name = variable with name that calculates value of bparam
       *kspace* arg = v_name
         v_name = variable with name that calculates scale factor on K-space terms
       *atom* args = aparam v_name
         aparam = parameter to adapt over time
         v_name = variable with name that calculates value of aparam

* zero or more keyword/value pairs may be appended
* keyword = *scale* or *reset*
  
  .. parsed-literal::
  
       *scale* value = *no* or *yes*
         *no* = the variable value is the new setting
         *yes* = the variable value multiplies the original setting
       *reset* value = *no* or *yes*
         *no* = values will remain altered at the end of a run
         *yes* = reset altered values to their original values at the end of a run



Examples
""""""""


.. parsed-literal::

   fix 1 all adapt 1 pair soft a 1 1 v_prefactor
   fix 1 all adapt 1 pair soft a 2\* 3 v_prefactor
   fix 1 all adapt 1 pair lj/cut epsilon \* \* v_scale1 coul/cut scale 3 3 v_scale2 scale yes reset yes
   fix 1 all adapt 10 atom diameter v_size

   variable ramp_up equal "ramp(0.01,0.5)"
   fix stretch all adapt 1 bond harmonic r0 1 v_ramp_up

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

Change or adapt one or more specific simulation attributes or settings
over time as a simulation runs.  Pair potential and K-space and atom
attributes which can be varied by this fix are discussed below.  Many
other fixes can also be used to time-vary simulation parameters,
e.g. the "fix deform" command will change the simulation box
size/shape and the "fix move" command will change atom positions and
velocities in a prescribed manner.  Also note that many commands allow
variables as arguments for specific parameters, if described in that
manner on their doc pages.  An equal-style variable can calculate a
time-dependent quantity, so this is another way to vary a simulation
parameter over time.

If *N* is specified as 0, the specified attributes are only changed
once, before the simulation begins.  This is all that is needed if the
associated variables are not time-dependent.  If *N* > 0, then changes
are made every *N* steps during the simulation, presumably with a
variable that is time-dependent.

Depending on the value of the *reset* keyword, attributes changed by
this fix will or will not be reset back to their original values at
the end of a simulation.  Even if *reset* is specified as *yes*\ , a
restart file written during a simulation will contain the modified
settings.

If the *scale* keyword is set to *no*\ , then the value the parameter is
set to will be whatever the variable generates.  If the *scale*
keyword is set to *yes*\ , then the value of the altered parameter will
be the initial value of that parameter multiplied by whatever the
variable generates.  I.e. the variable is now a "scale factor" applied
in (presumably) a time-varying fashion to the parameter.

Note that whether scale is *no* or *yes*\ , internally, the parameters
themselves are actually altered by this fix.  Make sure you use the
*reset yes* option if you want the parameters to be restored to their
initial values after the run.


----------


The *pair* keyword enables various parameters of potentials defined by
the :doc:`pair\_style <pair_style>` command to be changed, if the pair
style supports it.  Note that the :doc:`pair\_style <pair_style>` and
:doc:`pair\_coeff <pair_coeff>` commands must be used in the usual manner
to specify these parameters initially; the fix adapt command simply
overrides the parameters.

The *pstyle* argument is the name of the pair style.  If :doc:`pair\_style hybrid or hybrid/overlay <pair_hybrid>` is used, *pstyle* should be
a sub-style name.  If there are multiple sub-styles using the same
pair style, then *pstyle* should be specified as "style:N" where N is
which instance of the pair style you wish to adapt, e.g. the first,
second, etc.  For example, *pstyle* could be specified as "soft" or
"lubricate" or "lj/cut:1" or "lj/cut:2".  The *pparam* argument is the
name of the parameter to change.  This is the current list of pair
styles and parameters that can be varied by this fix.  See the doc
pages for individual pair styles and their energy formulas for the
meaning of these parameters:

+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`born <pair_born>`                                             | a,b,c                                            | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`born/coul/long, born/coul/msm <pair_born>`                    | coulombic\_cutoff                                | type global |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`buck <pair_buck>`                                             | a,c                                              | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`buck/coul/long, buck/coul/msm <pair_buck>`                    | coulombic\_cutoff                                | type global |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`buck/mdf <pair_mdf>`                                          | a,c                                              | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`coul/cut <pair_coul>`                                         | scale                                            | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`coul/cut/soft <pair_fep_soft>`                                | lambda                                           | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`coul/debye <pair_coul>`                                       | scale                                            | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`coul/dsf <pair_coul>`                                         | coulombic\_cutoff                                | type global |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`coul/long, coul/msm <pair_coul>`                              | coulombic\_cutoff, scale                         | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`coul/long/soft <pair_fep_soft>`                               | scale, lambda, coulombic\_cutoff                 | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`eam, eam/alloy, eam/fs <pair_eam>`                            | scale                                            | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`gauss <pair_gauss>`                                           | a                                                | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`lennard/mdf <pair_mdf>`                                       | A,B                                              | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`lj/class2 <pair_class2>`                                      | epsilon,sigma                                    | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`lj/class2/coul/cut, lj/class2/coul/long <pair_class2>`        | epsilon,sigma,coulombic\_cutoff                  | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`lj/cut <pair_lj>`                                             | epsilon,sigma                                    | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`lj/cut/coul/cut, lj/cut/coul/long, lj/cut/coul/msm <pair_lj>` | epsilon,sigma,coulombic\_cutoff                  | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`lj/cut/coul/cut/soft, lj/cut/coul/long/soft <pair_fep_soft>`  | epsilon,sigma,lambda,coulombic\_cutoff           | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`lj/cut/coul/dsf <pair_lj>`                                    | cutoff                                           | type global |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`lj/cut/tip4p/cut <pair_lj>`                                   | epsilon,sigma,coulombic\_cutoff                  | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`lj/cut/soft <pair_fep_soft>`                                  | epsilon,sigma,lambda                             | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`lj/expand <pair_lj_expand>`                                   | epsilon,sigma,delta                              | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`lj/mdf <pair_mdf>`                                            | epsilon,sigma                                    | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`lj/sf/dipole/sf <pair_dipole>`                                | epsilon,sigma,scale                              | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`lubricate <pair_lubricate>`                                   | mu                                               | global      |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`mie/cut <pair_mie>`                                           | epsilon,sigma,gamma\_repulsive,gamma\_attractive | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`morse, morse/smooth/linear <pair_morse>`                      | D0,R0,alpha                                      | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`morse/soft <pair_morse>`                                      | D0,R0,alpha,lambda                               | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`nm/cut <pair_nm>`                                             | E0,R0,m,n                                        | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`nm/cut/coul/cut, nm/cut/coul/long <pair_nm>`                  | E0,R0,m,n,coulombic\_cutoff                      | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`reax/c <pair_reaxc>`                                          | chi, eta, gamma                                  | type global |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`spin/dmi <pair_spin_dmi>`                                     | coulombic\_cutoff                                | type global |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`spin/exchange <pair_spin_exchange>`                           | coulombic\_cutoff                                | type global |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`spin/magelec <pair_spin_magelec>`                             | coulombic\_cutoff                                | type global |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`spin/neel <pair_spin_neel>`                                   | coulombic\_cutoff                                | type global |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`table <pair_table>`                                           | table\_cutoff                                    | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`ufm <pair_ufm>`                                               | epsilon,sigma                                    | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
| :doc:`soft <pair_soft>`                                             | a                                                | type pairs  |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+
|                                                                     |                                                  |             |
+---------------------------------------------------------------------+--------------------------------------------------+-------------+

.. note::

   It is easy to add new pairwise potentials and their parameters
   to this list.  All it typically takes is adding an extract() method to
   the pair\_\*.cpp file associated with the potential.

Some parameters are global settings for the pair style, e.g. the
viscosity setting "mu" for :doc:`pair\_style lubricate <pair_lubricate>`.
Other parameters apply to atom type pairs within the pair style,
e.g. the prefactor "a" for :doc:`pair\_style soft <pair_soft>`.

Note that for many of the potentials, the parameter that can be varied
is effectively a prefactor on the entire energy expression for the
potential, e.g. the lj/cut epsilon.  The parameters listed as "scale"
are exactly that, since the energy expression for the
:doc:`coul/cut <pair_coul>` potential (for example) has no labeled
prefactor in its formula.  To apply an effective prefactor to some
potentials, multiple parameters need to be altered.  For example, the
:doc:`Buckingham potential <pair_buck>` needs both the A and C terms
altered together.  To scale the Buckingham potential, you should thus
list the pair style twice, once for A and once for C.

If a type pair parameter is specified, the *I* and *J* settings should
be specified to indicate which type pairs to apply it to.  If a global
parameter is specified, the *I* and *J* settings still need to be
specified, but are ignored.

Similar to the :doc:`pair\_coeff command <pair_coeff>`, I and J can be
specified in one of two ways.  Explicit numeric values can be used for
each, as in the 1st example above.  I <= J is required.  LAMMPS sets
the coefficients for the symmetric J,I interaction to the same values.

A wild-card asterisk can be used in place of or in conjunction with
the I,J arguments to set the coefficients for multiple pairs of atom
types.  This takes the form "\*" or "\*n" or "n\*" or "m\*n".  If N = the
number of atom types, then an asterisk with no numeric values means
all types from 1 to N.  A leading asterisk means all types from 1 to n
(inclusive).  A trailing asterisk means all types from n to N
(inclusive).  A middle asterisk means all types from m to n
(inclusive).  Note that only type pairs with I <= J are considered; if
asterisks imply type pairs where J < I, they are ignored.

IMPROTANT NOTE: If :doc:`pair\_style hybrid or hybrid/overlay <pair_hybrid>` is being used, then the *pstyle* will
be a sub-style name.  You must specify I,J arguments that correspond
to type pair values defined (via the :doc:`pair\_coeff <pair_coeff>`
command) for that sub-style.

The *v\_name* argument for keyword *pair* is the name of an
:doc:`equal-style variable <variable>` which will be evaluated each time
this fix is invoked to set the parameter to a new value.  It should be
specified as v\_name, where name is the variable name.  Equal-style
variables can specify formulas with various mathematical functions,
and include :doc:`thermo\_style <thermo_style>` command keywords for the
simulation box parameters and timestep and elapsed time.  Thus it is
easy to specify parameters that change as a function of time or span
consecutive runs in a continuous fashion.  For the latter, see the
*start* and *stop* keywords of the :doc:`run <run>` command and the
*elaplong* keyword of :doc:`thermo\_style custom <thermo_style>` for
details.

For example, these commands would change the prefactor coefficient of
the :doc:`pair\_style soft <pair_soft>` potential from 10.0 to 30.0 in a
linear fashion over the course of a simulation:


.. parsed-literal::

   variable prefactor equal ramp(10,30)
   fix 1 all adapt 1 pair soft a \* \* v_prefactor


----------


The *bond* keyword uses the specified variable to change the value of
a bond coefficient over time, very similar to how the *pair* keyword
operates. The only difference is that now a bond coefficient for a
given bond type is adapted.

A wild-card asterisk can be used in place of or in conjunction with
the bond type argument to set the coefficients for multiple bond types.
This takes the form "\*" or "\*n" or "n\*" or "m\*n".  If N = the number of
atom types, then an asterisk with no numeric values means all types
from 1 to N.  A leading asterisk means all types from 1 to n (inclusive).
A trailing asterisk means all types from n to N (inclusive).  A middle
asterisk means all types from m to n (inclusive).

Currently *bond* does not support bond\_style hybrid nor bond\_style
hybrid/overlay as bond styles. The only bonds that currently are
working with fix\_adapt are

+---------------------------------+-------+------------+
| :doc:`gromos <bond_gromos>`     | k, r0 | type bonds |
+---------------------------------+-------+------------+
| :doc:`harmonic <bond_harmonic>` | k,r0  | type bonds |
+---------------------------------+-------+------------+


----------


The *kspace* keyword used the specified variable as a scale factor on
the energy, forces, virial calculated by whatever K-Space solver is
defined by the :doc:`kspace\_style <kspace_style>` command.  If the
variable has a value of 1.0, then the solver is unaltered.

The *kspace* keyword works this way whether the *scale* keyword
is set to *no* or *yes*\ .


----------


The *atom* keyword enables various atom properties to be changed.  The
*aparam* argument is the name of the parameter to change.  This is the
current list of atom parameters that can be varied by this fix:

* charge = charge on particle
* diameter = diameter of particle

The *v\_name* argument of the *atom* keyword is the name of an
:doc:`equal-style variable <variable>` which will be evaluated each time
this fix is invoked to set the parameter to a new value.  It should be
specified as v\_name, where name is the variable name.  See the
discussion above describing the formulas associated with equal-style
variables.  The new value is assigned to the corresponding attribute
for all atoms in the fix group.

.. note::

   The *atom* keyword works this way whether the *scale* keyword is
   set to *no* or *yes*\ .  I.e. the use of scale yes is not yet supported
   by the *atom* keyword.

If the atom parameter is *diameter* and per-atom density and per-atom
mass are defined for particles (e.g. :doc:`atom\_style granular <atom_style>`), then the mass of each particle is also
changed when the diameter changes (density is assumed to stay
constant).

For example, these commands would shrink the diameter of all granular
particles in the "center" group from 1.0 to 0.1 in a linear fashion
over the course of a 1000-step simulation:


.. parsed-literal::

   variable size equal ramp(1.0,0.1)
   fix 1 center adapt 10 atom diameter v_size


----------


**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>`.

For :doc:`rRESPA time integration <run_style>`, this fix changes
parameters on the outermost rRESPA level.

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

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

:doc:`compute ti <compute_ti>`

Default
"""""""

The option defaults are scale = no, reset = no.


.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Commands_all.html
