.. index:: fix move fix move command ================ Syntax """""" .. parsed-literal:: fix ID group-ID move style args keyword values ... * ID, group-ID are documented in :doc:`fix ` command * move = style name of this fix command * style = *linear* or *wiggle* or *rotate* 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) *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 """""""" .. parsed-literal:: 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 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. .. warning:: The atoms affected by this fix should not normally be time integrated by other fixes (e.g. :doc:`fix nve `, :doc:`fix nvt `), since that will change their positions and velocities twice. .. warning:: As atoms move due to this fix, they will pass thru 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 ` 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. .. warning:: As dicsussed 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 ` command for a discussion of "unwrapped" coordinates. See the Atoms section of the :doc:`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 ` 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 ` 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 ` that uses the vdisplace() function. E.g. .. parsed-literal:: variable V equal 10.0 variable x equal vdisplace(0.0,$V) fix 1 boundary move variable v_x NULL NULL v_V NULL NULL The *wiggle* style moves atoms in an oscillatory fashion, so that their position *X* = (x,y,z) as a function of time is given in vector notation as .. parsed-literal:: X(t) = X0 + A sin(omega*delta) where *X0* = (x0,y0,z0) is their position at the time the fix is specified, *A* is the specified amplitude vector with components (Ax,Ay,Az), *omega* is 2 PI / *period*, and *delta* is the time elapsed since the fix was specified. This style also sets the velocity of each atom to the time derivative of this expression. If any of the amplitude components is specified as NULL, then the position and velocity of that component is time integrated the same as the :doc:`fix nve ` command would perform, using the corresponding force component on the atom. Note that the *wiggle* style is identical to using the *variable* style with :doc:`equal-style variables ` that use the swiggle() and cwiggle() functions. E.g. .. parsed-literal:: variable A equal 10.0 variable T equal 5.0 variable omega equal 2.0*PI/$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 thru a point *P* = (Px,Py,Pz). The *period* of the rotation is also specified. 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 ` assigns an angular velocity to each atom, then each atom's angular velocity is also set to omega. Note that the direction of rotation for the atoms around the rotation axis is consistent with the right-hand rule: if your right-hand's thumb points along *R*, then your fingers wrap around the axis in the direction of rotation. The *variable* style allows the position and velocity components of each atom to be set by formulas specified via the :doc:`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 ` 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 ` 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 ` command, e.g. velocity in Angstroms/fmsec 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 ` 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. For :doc:`rRESPA time integration `, this fix adjusts the position and velocity of atoms on the outermost rRESPA level. ---------- Restart, fix_modify, output, run start/stop, minimize info """""""""""""""""""""""""""""""""""""""""""""""""""""""""" This fix writes the original coordinates of moving atoms to :doc:`binary restart files `, as well as the initial timestep, so that the motion can be continuous in a restarted simulation. See the :doc:`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. IMPORTANNT 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 ` options are relevant to this fix. This fix produces a per-atom array which can be accessed by various :ref:`output commands `. 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 ` command. This fix is not invoked during :doc:`energy minimization `. Restrictions """""""""""" none Related commands """""""""""""""" :doc:`fix nve `, :doc:`displace_atoms ` **Default:** none The option default is units = box. .. _liws: http://www.cfdem.com .. _ld: Manual.html .. _lc: Section_commands.html#comm