forked from lijiext/lammps
235 lines
11 KiB
HTML
235 lines
11 KiB
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<HR>
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<H3>fix move command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID move style args keyword values ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>move = style name of this fix command
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<LI>style = <I>linear</I> or <I>wiggle</I> or <I>rotate</I> or <I>variable</I>
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<PRE> <I>linear</I> args = Vx Vy Vz
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Vx,Vy,Vz = components of velocity vector (velocity units), any component can be specified as NULL
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<I>wiggle</I> args = Ax Ay Az period
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Ax,Ay,Az = components of amplitude vector (distance units), any component can be specified as NULL
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period = period of oscillation (time units)
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<I>rotate</I> args = Px Py Pz Rx Ry Rz period
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Px,Py,Pz = origin point of axis of rotation (distance units)
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Rx,Ry,Rz = axis of rotation vector
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period = period of rotation (time units)
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<I>variable</I> args = v_dx v_dy v_dz v_vx v_vy v_vz
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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
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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
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</PRE>
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<LI>zero or more keyword/value pairs may be appended
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<LI>keyword = <I>units</I>
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<PRE> <I>units</I> value = <I>box</I> or <I>lattice</I>
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 1 boundary move wiggle 3.0 0.0 0.0 1.0 units box
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fix 2 boundary move rotate 0.0 0.0 0.0 0.0 0.0 1.0 5.0
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fix 2 boundary move variable v_myx v_myy NULL v_VX v_VY NULL
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Perform updates of position and velocity for atoms in the group each
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timestep using the specified settings or formulas, without regard to
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forces on the atoms. This can be useful for boundary or other atoms,
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whose movement can influence nearby atoms.
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</P>
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<P>IMPORTANT NOTE: The atoms affected by this fix should not normally be
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time integrated by other fixes (e.g. <A HREF = "fix_nve.html">fix nve</A>, <A HREF = "fix_nh.html">fix
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nvt</A>), since that will change their positions and
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velocities twice.
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</P>
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<P>IMPORTANT NOTE: As atoms move due to this fix, they will pass thru
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periodic boundaries and be remapped to the other side of the
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simulation box, just as they would during normal time integration
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(e.g. via the <A HREF = "fix_nve.html">fix nve</A> command). It is up to you to
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decide whether periodic boundaries are appropriate with the kind of
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atom motion you are prescribing with this fix.
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</P>
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<P>IMPORTANT NOTE: As dicsussed below, atoms are moved relative to their
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initial position at the time the fix is specified. These initial
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coordinates are stored by the fix in "unwrapped" form, by using the
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image flags associated with each atom. See the <A HREF = "dump.html">dump
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custom</A> command for a discussion of "unwrapped" coordinates.
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See the Atoms section of the <A HREF = "read_data.html">read_data</A> command for a
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discussion of image flags and how they are set for each atom. You can
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reset the image flags (e.g. to 0) before invoking this fix by using
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the <A HREF = "set.html">set image</A> command.
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</P>
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<HR>
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<P>The <I>linear</I> style moves atoms at a constant velocity, so that their
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position <I>X</I> = (x,y,z) as a function of time is given in vector
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notation as
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</P>
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<PRE>X(t) = X0 + V * delta
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</PRE>
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<P>where <I>X0</I> = (x0,y0,z0) is their position at the time the fix is
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specified, <I>V</I> is the specified velocity vector with components
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(Vx,Vy,Vz), and <I>delta</I> is the time elapsed since the fix was
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specified. This style also sets the velocity of each atom to V =
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(Vx,Vy,Vz). If any of the velocity components is specified as NULL,
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then the position and velocity of that component is time integrated
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the same as the <A HREF = "fix_nve.html">fix nve</A> command would perform, using
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the corresponding force component on the atom.
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</P>
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<P>Note that the <I>linear</I> style is identical to using the <I>variable</I>
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style with an <A HREF = "variable.html">equal-style variable</A> that uses the
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vdisplace() function. E.g.
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</P>
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<PRE>variable V equal 10.0
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variable x equal vdisplace(0.0,$V)
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fix 1 boundary move variable v_x NULL NULL v_V NULL NULL
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</PRE>
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<P>The <I>wiggle</I> style moves atoms in an oscillatory fashion, so that
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their position <I>X</I> = (x,y,z) as a function of time is given in vector
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notation as
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</P>
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<PRE>X(t) = X0 + A sin(omega*delta)
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</PRE>
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<P>where <I>X0</I> = (x0,y0,z0) is their position at the time the fix is
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specified, <I>A</I> is the specified amplitude vector with components
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(Ax,Ay,Az), <I>omega</I> is 2 PI / <I>period</I>, and <I>delta</I> is the time
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elapsed since the fix was specified. This style also sets the
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velocity of each atom to the time derivative of this expression. If
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any of the amplitude components is specified as NULL, then the
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position and velocity of that component is time integrated the same as
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the <A HREF = "fix_nve.html">fix nve</A> command would perform, using the
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corresponding force component on the atom.
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</P>
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<P>Note that the <I>wiggle</I> style is identical to using the <I>variable</I>
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style with <A HREF = "variable.html">equal-style variables</A> that use the
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swiggle() and cwiggle() functions. E.g.
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</P>
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<PRE>variable A equal 10.0
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variable T equal 5.0
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variable omega equal 2.0*PI/$T
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variable x equal swiggle(0.0,$A,$T)
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variable v equal v_omega*($A-cwiggle(0.0,$A,$T))
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fix 1 boundary move variable v_x NULL NULL v_v NULL NULL
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</PRE>
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<P>The <I>rotate</I> style rotates atoms around a rotation axis <I>R</I> =
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(Rx,Ry,Rz) that goes thru a point <I>P</I> = (Px,Py,Pz). The <I>period</I> of
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the rotation is also specified. This style also sets the velocity of
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each atom to (omega cross Rperp) where omega is its angular velocity
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around the rotation axis and Rperp is a perpendicular vector from the
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rotation axis to the atom. If the defined
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<A HREF = "atom_style.html">atom_style</A> assigns an angular velocity to each atom,
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then each atom's angular velocity is also set to omega. Note that the
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direction of rotation for the atoms around the rotation axis is
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consistent with the right-hand rule: if your right-hand's thumb points
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along <I>R</I>, then your fingers wrap around the axis in the direction of
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rotation.
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</P>
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<P>The <I>variable</I> style allows the position and velocity components of
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each atom to be set by formulas specified via the
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<A HREF = "variable.html">variable</A> command. Each of the 6 variables is
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specified as an argument to the fix as v_name, where name is the
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variable name that is defined elsewhere in the input script.
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</P>
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<P>Each variable must be of either the <I>equal</I> or <I>atom</I> style.
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<I>Equal</I>-style variables compute a single numeric quantity, that can be
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a function of the timestep as well as of other simulation values.
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<I>Atom</I>-style variables compute a numeric quantity for each atom, that
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can be a function per-atom quantities, such as the atom's position, as
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well as of the timestep and other simulation values. Note that this
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fix stores the original coordinates of each atom (see note below) so
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that per-atom quantity can be used in an atom-style variable formula.
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See the <A HREF = "variable.html">variable</A> command for details.
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</P>
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<P>The first 3 variables (v_dx,v_dy,v_dz) specified for the <I>variable</I>
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style are used to calculate a displacement from the atom's original
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position at the time the fix was specified. The second 3 variables
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(v_vx,v_vy,v_vz) specified are used to compute a velocity for each
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atom.
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</P>
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<P>Any of the 6 variables can be specified as NULL. If both the
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displacement and velocity variables for a particular x,y,z component
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are specified as NULL, then the position and velocity of that
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component is time integrated the same as the <A HREF = "fix_nve.html">fix nve</A>
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command would perform, using the corresponding force component on the
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atom. If only the velocity variable for a component is specified as
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NULL, then the displacement variable will be used to set the position
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of the atom, and its velocity component will not be changed. If only
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the displacement variable for a component is specified as NULL, then
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the velocity variable will be used to set the velocity of the atom,
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and the position of the atom will be time integrated using that
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velocity.
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</P>
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<P>The <I>units</I> keyword determines the meaning of the distance units used
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to define the <I>linear</I> velocity and <I>wiggle</I> amplitude and <I>rotate</I>
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origin. This setting is ignored for the <I>variable</I> style. A <I>box</I>
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value selects standard units as defined by the <A HREF = "units.html">units</A>
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command, e.g. velocity in Angstroms/fmsec and amplitude and position
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in Angstroms for units = real. A <I>lattice</I> value means the velocity
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units are in lattice spacings per time and the amplitude and position
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are in lattice spacings. The <A HREF = "lattice.html">lattice</A> command must have
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been previously used to define the lattice spacing. Each of these 3
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quantities may be dependent on the x,y,z dimension, since the lattice
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spacings can be different in x,y,z.
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</P>
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<HR>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>This fix writes the original coordinates of moving atoms to <A HREF = "restart.html">binary
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restart files</A>, as well as the initial timestep, so that
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the motion can be continuous in a restarted simulation. See the
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<A HREF = "read_restart.html">read_restart</A> command for info on how to re-specify
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a fix in an input script that reads a restart file, so that the
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operation of the fix continues in an uninterrupted fashion.
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</P>
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<P>IMPORTANNT NOTE: Because the move positions are a function of the
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current timestep and the initial timestep, you cannot reset the
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timestep to a different value after reading a restart file, if you
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expect a fix move command to work in an uninterrupted fashion.
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</P>
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<P>None of the <A HREF = "fix_modify.html">fix_modify</A> options are relevant to this
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fix.
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</P>
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<P>This fix produces a per-atom array which can be accessed by various
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<A HREF = "Section_howto.html#howto_15">output commands</A>. The number of columns
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for each atom is 3, and the columns store the original unwrapped x,y,z
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coords of each atom. The per-atom values can be accessed on any
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timestep.
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</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
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minimization</A>.
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</P>
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<P>For <A HREF = "run_style.html">rRESPA time integration</A>, this fix adjusts the
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position and velocity of atoms on the outermost rRESPA level.
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</P>
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<P><B>Restrictions:</B> none
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "fix_nve.html">fix nve</A>, <A HREF = "displace_atoms.html">displace_atoms</A>
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</P>
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<P><B>Default:</B> none
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</P>
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<P>The option default is units = lattice.
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</P>
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</HTML>
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