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<div class="section" id="fix-move-command">
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<span id="index-0"></span><h1>fix move command<a class="headerlink" href="#fix-move-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="syntax">
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<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID move style args keyword values ...
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</pre></div>
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</div>
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<ul class="simple">
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<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</li>
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<li>move = style name of this fix command</li>
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<li>style = <em>linear</em> or <em>wiggle</em> or <em>rotate</em> or <em>variable</em></li>
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</ul>
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<pre class="literal-block">
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<em>linear</em> 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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<em>wiggle</em> 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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<em>rotate</em> 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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<em>variable</em> 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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<ul class="simple">
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<li>zero or more keyword/value pairs may be appended</li>
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<li>keyword = <em>units</em></li>
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</ul>
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<pre class="literal-block">
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<em>units</em> value = <em>box</em> or <em>lattice</em>
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</pre>
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</div>
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<div class="section" id="examples">
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<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><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></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
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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.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">The atoms affected by this fix should not normally be time
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integrated by other fixes (e.g. <a class="reference internal" href="fix_nve.html"><em>fix nve</em></a>, <a class="reference internal" href="fix_nh.html"><em>fix nvt</em></a>), since that will change their positions and
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velocities twice.</p>
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</div>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">As atoms move due to this fix, they will pass thru periodic
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boundaries and be remapped to the other side of the simulation box,
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just as they would during normal time integration (e.g. via the <a class="reference internal" href="fix_nve.html"><em>fix nve</em></a> command). It is up to you to decide whether
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periodic boundaries are appropriate with the kind of atom motion you
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are prescribing with this fix.</p>
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</div>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">As dicsussed below, atoms are moved relative to their initial
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position at the time the fix is specified. These initial coordinates
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are stored by the fix in “unwrapped” form, by using the image flags
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associated with each atom. See the <a class="reference internal" href="dump.html"><em>dump custom</em></a> command
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for a discussion of “unwrapped” coordinates. See the Atoms section of
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the <a class="reference internal" href="read_data.html"><em>read_data</em></a> command for a discussion of image flags
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and how they are set for each atom. You can reset the image flags
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(e.g. to 0) before invoking this fix by using the <a class="reference internal" href="set.html"><em>set image</em></a>
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command.</p>
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</div>
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<hr class="docutils" />
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<p>The <em>linear</em> style moves atoms at a constant velocity, so that their
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position <em>X</em> = (x,y,z) as a function of time is given in vector
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notation as</p>
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<div class="highlight-python"><div class="highlight"><pre><span class="n">X</span><span class="p">(</span><span class="n">t</span><span class="p">)</span> <span class="o">=</span> <span class="n">X0</span> <span class="o">+</span> <span class="n">V</span> <span class="o">*</span> <span class="n">delta</span>
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</pre></div>
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</div>
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<p>where <em>X0</em> = (x0,y0,z0) is their position at the time the fix is
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specified, <em>V</em> is the specified velocity vector with components
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(Vx,Vy,Vz), and <em>delta</em> 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 class="reference internal" href="fix_nve.html"><em>fix nve</em></a> command would perform, using
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the corresponding force component on the atom.</p>
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<p>Note that the <em>linear</em> style is identical to using the <em>variable</em>
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style with an <a class="reference internal" href="variable.html"><em>equal-style variable</em></a> that uses the
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vdisplace() function. E.g.</p>
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<div class="highlight-python"><div class="highlight"><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></div>
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</div>
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<p>The <em>wiggle</em> style moves atoms in an oscillatory fashion, so that
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their position <em>X</em> = (x,y,z) as a function of time is given in vector
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notation as</p>
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<div class="highlight-python"><div class="highlight"><pre>X(t) = X0 + A sin(omega*delta)
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</pre></div>
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</div>
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<p>where <em>X0</em> = (x0,y0,z0) is their position at the time the fix is
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specified, <em>A</em> is the specified amplitude vector with components
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(Ax,Ay,Az), <em>omega</em> is 2 PI / <em>period</em>, and <em>delta</em> 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 class="reference internal" href="fix_nve.html"><em>fix nve</em></a> command would perform, using the
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corresponding force component on the atom.</p>
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<p>Note that the <em>wiggle</em> style is identical to using the <em>variable</em>
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style with <a class="reference internal" href="variable.html"><em>equal-style variables</em></a> that use the
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swiggle() and cwiggle() functions. E.g.</p>
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<div class="highlight-python"><div class="highlight"><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></div>
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</div>
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<p>The <em>rotate</em> style rotates atoms around a rotation axis <em>R</em> =
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(Rx,Ry,Rz) that goes thru a point <em>P</em> = (Px,Py,Pz). The <em>period</em> of
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the rotation is also specified. The direction of rotation for the
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atoms around the rotation axis is consistent with the right-hand rule:
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if your right-hand thumb points along <em>R</em>, then your fingers wrap
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around the axis in the direction of rotation.</p>
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<p>This style also sets the velocity of each atom to (omega cross Rperp)
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where omega is its angular velocity around the rotation axis and Rperp
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is a perpendicular vector from the rotation axis to the atom. If the
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defined <a class="reference internal" href="atom_style.html"><em>atom_style</em></a> assigns an angular velocity or
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angular moementum or orientation to each atom (<a class="reference internal" href="atom_style.html"><em>atom styles</em></a> sphere, ellipsoid, line, tri, body), then
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those properties are also updated appropriately to correspond to the
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atom’s motion and rotation over time.</p>
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<p>The <em>variable</em> 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 class="reference internal" href="variable.html"><em>variable</em></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.</p>
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<p>Each variable must be of either the <em>equal</em> or <em>atom</em> style.
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<em>Equal</em>-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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<em>Atom</em>-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 class="reference internal" href="variable.html"><em>variable</em></a> command for details.</p>
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<p>The first 3 variables (v_dx,v_dy,v_dz) specified for the <em>variable</em>
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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.</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 class="reference internal" href="fix_nve.html"><em>fix nve</em></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.</p>
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<p>The <em>units</em> keyword determines the meaning of the distance units used
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to define the <em>linear</em> velocity and <em>wiggle</em> amplitude and <em>rotate</em>
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origin. This setting is ignored for the <em>variable</em> style. A <em>box</em>
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value selects standard units as defined by the <a class="reference internal" href="units.html"><em>units</em></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 <em>lattice</em> 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 class="reference internal" href="lattice.html"><em>lattice</em></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.</p>
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</div>
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<hr class="docutils" />
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<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
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<h2>Restart, fix_modify, output, run start/stop, minimize info<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Permalink to this headline">¶</a></h2>
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<p>This fix writes the original coordinates of moving atoms to <a class="reference internal" href="restart.html"><em>binary restart files</em></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 class="reference internal" href="read_restart.html"><em>read_restart</em></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.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">Because the move positions are a function of the current
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timestep and the initial timestep, you cannot reset the timestep to a
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different value after reading a restart file, if you expect a fix move
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command to work in an uninterrupted fashion.</p>
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</div>
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<p>None of the <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> options are relevant to this
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fix.</p>
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<p>This fix produces a per-atom array which can be accessed by various
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<a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></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.</p>
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<p>No parameter of this fix can be used with the <em>start/stop</em> keywords of
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the <a class="reference internal" href="run.html"><em>run</em></a> command. This fix is not invoked during <a class="reference internal" href="minimize.html"><em>energy minimization</em></a>.</p>
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<p>For <a class="reference internal" href="run_style.html"><em>rRESPA time integration</em></a>, this fix adjusts the
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position and velocity of atoms on the outermost rRESPA level.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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<blockquote>
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<div>none</div></blockquote>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
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<p><a class="reference internal" href="fix_nve.html"><em>fix nve</em></a>, <a class="reference internal" href="displace_atoms.html"><em>displace_atoms</em></a></p>
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<p><strong>Default:</strong> none</p>
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<p>The option default is units = lattice.</p>
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