.. index:: fix move

fix move command
================

Syntax
""""""

.. code-block:: LAMMPS

   fix ID group-ID move style args keyword values ...

* ID, group-ID are documented in :doc:`fix <fix>` command
* move = style name of this fix command
* style = *linear* or *wiggle* or *rotate* or *transrot* or *variable*

  .. parsed-literal::

       *linear* args = Vx Vy Vz
         Vx,Vy,Vz = components of velocity vector (velocity units), any component can be specified as NULL
       *wiggle* args = Ax Ay Az period
         Ax,Ay,Az = components of amplitude vector (distance units), any component can be specified as NULL
         period = period of oscillation (time units)
       *rotate* args = Px Py Pz Rx Ry Rz period
         Px,Py,Pz = origin point of axis of rotation (distance units)
         Rx,Ry,Rz = axis of rotation vector
         period = period of rotation (time units)
       *transrot* args = Vx Vy Vz Px Py Pz Rx Ry Rz period
         Vx,Vy,Vz = components of velocity vector (velocity units)
         Px,Py,Pz = origin point of axis of rotation (distance units)
         Rx,Ry,Rz = axis of rotation vector
         period = period of rotation (time units)
       *variable* args = v_dx v_dy v_dz v_vx v_vy v_vz
         v_dx,v_dy,v_dz = 3 variable names that calculate x,y,z displacement as function of time, any component can be specified as NULL
         v_vx,v_vy,v_vz = 3 variable names that calculate x,y,z velocity as function of time, any component can be specified as NULL

* zero or more keyword/value pairs may be appended
* keyword = *units*

  .. parsed-literal::

       *units* value = *box* or *lattice*

Examples
""""""""

.. code-block:: LAMMPS

   fix 1 boundary move wiggle 3.0 0.0 0.0 1.0 units box
   fix 2 boundary move rotate 0.0 0.0 0.0 0.0 0.0 1.0 5.0
   fix 2 boundary move variable v_myx v_myy NULL v_VX v_VY NULL
   fix 3 boundary move transrot 0.1 0.1 0.0 0.0 0.0 0.0 0.0 0.0 1.0 5.0 units box

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

Perform updates of position and velocity for atoms in the group each
timestep using the specified settings or formulas, without regard to
forces on the atoms.  This can be useful for boundary or other atoms,
whose movement can influence nearby atoms.

.. note::

   The atoms affected by this fix should not normally be time integrated
   by other fixes (e.g. :doc:`fix nve <fix_nve>`, :doc:`fix nvt
   <fix_nh>`), since that will change their positions and velocities
   twice.

.. note::

   As atoms move due to this fix, they will pass through periodic
   boundaries and be remapped to the other side of the simulation box,
   just as they would during normal time integration (e.g. via the
   :doc:`fix nve <fix_nve>` command).  It is up to you to decide whether
   periodic boundaries are appropriate with the kind of atom motion you
   are prescribing with this fix.

.. note::

   As discussed below, atoms are moved relative to their initial
   position at the time the fix is specified.  These initial coordinates
   are stored by the fix in "unwrapped" form, by using the image flags
   associated with each atom.  See the :doc:`dump custom <dump>` command
   for a discussion of "unwrapped" coordinates.  See the Atoms section
   of the :doc:`read_data <read_data>` command for a discussion of image
   flags and how they are set for each atom.  You can reset the image
   flags (e.g. to 0) before invoking this fix by using the :doc:`set
   image <set>` command.

----------

The *linear* style moves atoms at a constant velocity, so that their
position *X* = (x,y,z) as a function of time is given in vector
notation as

.. parsed-literal::

   X(t) = X0 + V \* delta

where *X0* = (x0,y0,z0) is their position at the time the fix is
specified, *V* is the specified velocity vector with components
(Vx,Vy,Vz), and *delta* is the time elapsed since the fix was
specified.  This style also sets the velocity of each atom to V =
(Vx,Vy,Vz).  If any of the velocity components is specified as NULL,
then the position and velocity of that component is time integrated
the same as the :doc:`fix nve <fix_nve>` command would perform, using
the corresponding force component on the atom.

Note that the *linear* style is identical to using the *variable*
style with an :doc:`equal-style variable <variable>` that uses the
vdisplace() function.  E.g.

.. code-block:: LAMMPS

   variable V equal 10.0
   variable x equal vdisplace(0.0,$V)
   fix 1 boundary move variable v_x NULL NULL v_V NULL NULL

The *wiggle* style moves atoms in an oscillatory fashion, so that
their position *X* = (x,y,z) as a function of time is given in vector
notation as

.. parsed-literal::

   X(t) = X0 + A sin(omega\*delta)

where *X0* = (x0,y0,z0) is their position at the time the fix is
specified, *A* is the specified amplitude vector with components
(Ax,Ay,Az), *omega* is 2 PI / *period*, and *delta* is the time elapsed
since the fix was specified.  This style also sets the velocity of each
atom to the time derivative of this expression.  If any of the amplitude
components is specified as NULL, then the position and velocity of that
component is time integrated the same as the :doc:`fix nve <fix_nve>`
command would perform, using the corresponding force component on the
atom.

Note that the *wiggle* style is identical to using the *variable*
style with :doc:`equal-style variables <variable>` that use the
swiggle() and cwiggle() functions.  E.g.

.. code-block:: LAMMPS

   variable A equal 10.0
   variable T equal 5.0
   variable omega equal 2.0*PI/$T
   variable x equal swiggle(0.0,$A,$T)
   variable v equal v_omega*($A-cwiggle(0.0,$A,$T))
   fix 1 boundary move variable v_x NULL NULL v_v NULL NULL

The *rotate* style rotates atoms around a rotation axis *R* = (Rx,Ry,Rz)
that goes through a point *P* = (Px,Py,Pz).  The *period* of the
rotation is also specified.  The direction of rotation for the atoms
around the rotation axis is consistent with the right-hand rule: if your
right-hand thumb points along *R*, then your fingers wrap around the
axis in the direction of rotation.

This style also sets the velocity of each atom to (omega cross Rperp)
where omega is its angular velocity around the rotation axis and Rperp
is a perpendicular vector from the rotation axis to the atom.  If the
defined :doc:`atom_style <atom_style>` assigns an angular velocity or
angular momentum or orientation to each atom (:doc:`atom styles
<atom_style>` sphere, ellipsoid, line, tri, body), then
those properties are also updated appropriately to correspond to the
atom's motion and rotation over time.

The *transrot* style combines the effects of *rotate* and *linear* so
that it is possible to prescribe a rotating group of atoms that also
moves at a constant velocity.  The arguments are for the translation
first and then for the rotation.  Since the rotation affects all
coordinate components, it is not possible to set any of the
translation vector components to NULL.

The *variable* style allows the position and velocity components of
each atom to be set by formulas specified via the
:doc:`variable <variable>` command.  Each of the 6 variables is
specified as an argument to the fix as v_name, where name is the
variable name that is defined elsewhere in the input script.

Each variable must be of either the *equal* or *atom* style.
*Equal*\ -style variables compute a single numeric quantity, that can be
a function of the timestep as well as of other simulation values.
*Atom*\ -style variables compute a numeric quantity for each atom, that
can be a function per-atom quantities, such as the atom's position, as
well as of the timestep and other simulation values.  Note that this fix
stores the original coordinates of each atom (see note below) so that
per-atom quantity can be used in an atom-style variable formula.  See
the :doc:`variable <variable>` command for details.

The first 3 variables (v_dx,v_dy,v_dz) specified for the *variable*
style are used to calculate a displacement from the atom's original
position at the time the fix was specified.  The second 3 variables
(v_vx,v_vy,v_vz) specified are used to compute a velocity for each
atom.

Any of the 6 variables can be specified as NULL.  If both the
displacement and velocity variables for a particular x,y,z component
are specified as NULL, then the position and velocity of that
component is time integrated the same as the :doc:`fix nve <fix_nve>`
command would perform, using the corresponding force component on the
atom.  If only the velocity variable for a component is specified as
NULL, then the displacement variable will be used to set the position
of the atom, and its velocity component will not be changed.  If only
the displacement variable for a component is specified as NULL, then
the velocity variable will be used to set the velocity of the atom,
and the position of the atom will be time integrated using that
velocity.

The *units* keyword determines the meaning of the distance units used
to define the *linear* velocity and *wiggle* amplitude and *rotate*
origin.  This setting is ignored for the *variable* style.  A *box*
value selects standard units as defined by the :doc:`units <units>`
command, e.g. velocity in Angstroms/fs and amplitude and position
in Angstroms for units = real.  A *lattice* value means the velocity
units are in lattice spacings per time and the amplitude and position
are in lattice spacings.  The :doc:`lattice <lattice>` command must have
been previously used to define the lattice spacing.  Each of these 3
quantities may be dependent on the x,y,z dimension, since the lattice
spacings can be different in x,y,z.

----------

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

This fix writes the original coordinates of moving atoms to :doc:`binary
restart files <restart>`, as well as the initial timestep, so that the
motion can be continuous in a restarted simulation.  See the
:doc:`read_restart <read_restart>` command for info on how to re-specify
a fix in an input script that reads a restart file, so that the
operation of the fix continues in an uninterrupted fashion.

.. note::

   Because the move positions are a function of the current
   timestep and the initial timestep, you cannot reset the timestep to a
   different value after reading a restart file, if you expect a fix move
   command to work in an uninterrupted fashion.

None of the :doc:`fix_modify <fix_modify>` options are relevant to this
fix.

This fix produces a per-atom array which can be accessed by various
:doc:`output commands <Howto_output>`.  The number of columns for each
atom is 3, and the columns store the original unwrapped x,y,z coords of
each atom.  The per-atom values can be accessed on any timestep.

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 adjusts the
position and velocity of atoms on the outermost rRESPA level.

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

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

:doc:`fix nve <fix_nve>`, :doc:`displace_atoms <displace_atoms>`

Default
"""""""

none

The option default is units = lattice.
