forked from lijiext/lammps
204 lines
8.6 KiB
HTML
204 lines
8.6 KiB
HTML
<HTML>
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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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</CENTER>
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<HR>
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<H3>fix wall/srd 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 wall/srd face arg ... keyword value ...
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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>wall/srd = style name of this fix command
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<LI>one or more face/arg pairs may be appended
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<LI>face = <I>xlo</I> or <I>xhi</I> or <I>ylo</I> or <I>yhi</I> or <I>zlo</I> or <I>zhi</I>
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<PRE> <I>xlo</I>,<I>ylo</I>,<I>zlo</I> arg = EDGE or constant or variable
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EDGE = current lo edge of simulation box
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constant = number like 0.0 or -30.0 (distance units)
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variable = <A HREF = "variable.html">equal-style variable</A> like v_x or v_wiggle
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<I>xhi</I>,<I>yhi</I>,<I>zhi</I> arg = EDGE or constant or variable
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EDGE = current hi edge of simulation box
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constant = number like 50.0 or 100.3 (distance units)
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variable = <A HREF = "variable.html">equal-style variable</A> like v_x or v_wiggle
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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>lattice</I> or <I>box</I>
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<I>lattice</I> = the wall position is defined in lattice units
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<I>box</I> = the wall position is defined in simulation box units
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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 xwalls all wall/srd xlo EDGE xhi EDGE
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fix walls all wall/srd xlo 0.0 ylo 10.0 units box
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fix top all wall/srd zhi v_pressdown
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Bound the simulation with one or more walls which interact with
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stochastic reaction dynamics (SRD) particles as slip (smooth) or
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no-slip (rough) flat surfaces. The wall interaction is actually
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invoked via the <A HREF = "fix_srd.html">fix srd</A> command, only on the group of
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SRD particles it defines, so the group setting for the fix wall/srd
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command is ignored.
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</P>
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<P>A particle/wall collision occurs if an SRD particle moves outside the
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wall on a timestep. This alters the position and velocity of the SRD
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particle and imparts a force to the wall.
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</P>
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<P>The <I>collision</I> and <I>Tsrd</I> settings specified via the <A HREF = "fix_srd.html">fix
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srd</A> command affect the SRD/wall collisions. A <I>slip</I>
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setting for the <I>collision</I> keyword means that the tangential
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component of the SRD particle momentum is preserved. Thus only a
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normal force is imparted to the wall. The normal component of the new
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SRD velocity is sampled from a Gaussian distribution at temperature
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<I>Tsrd</I>.
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</P>
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<P>For a <I>noslip</I> setting of the <I>collision</I> keyword, both the normal and
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tangential components of the new SRD velocity are sampled from a
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Gaussian distribution at temperature <I>Tsrd</I>. Additionally, a new
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tangential direction for the SRD velocity is chosen randomly. This
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collision style imparts both a normal and tangential force to the
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wall.
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</P>
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<P>Up to 6 walls or faces can be specified in a single command: <I>xlo</I>,
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<I>xhi</I>, <I>ylo</I>, <I>yhi</I>, <I>zlo</I>, <I>zhi</I>. A <I>lo</I> face reflects particles
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that move to a coordinate less than the wall position, back in the
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<I>hi</I> direction. A <I>hi</I> face reflects particles that move to a
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coordinate higher than the wall position, back in the <I>lo</I> direction.
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</P>
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<P>The position of each wall can be specified in one of 3 ways: as the
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EDGE of the simulation box, as a constant value, or as a variable. If
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EDGE is used, then the corresponding boundary of the current
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simulation box is used. If a numeric constant is specified then the
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wall is placed at that position in the appropriate dimension (x, y, or
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z). In both the EDGE and constant cases, the wall will never move.
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If the wall position is a variable, it should be specified as v_name,
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where name is an <A HREF = "variable.html">equal-style variable</A> name. In this
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case the variable is evaluated each timestep and the result becomes
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the current position of the reflecting wall. Equal-style variables
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can specify formulas with various mathematical functions, and include
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<A HREF = "thermo_style.html">thermo_style</A> command keywords for the simulation
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box parameters and timestep and elapsed time. Thus it is easy to
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specify a time-dependent wall position.
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</P>
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<P>IMPORTANT NOTE: Because the trajectory of the SRD particle is tracked
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as it collides with the wall, you must insure that r = distance of the
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particle from the wall, is always > 0 for SRD particles, or LAMMPS
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will generate an error. This means you cannot start your simulation
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with SRD particles at the wall position <I>coord</I> (r = 0) or with
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particles on the wrong side of the wall (r < 0).
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</P>
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<P>IMPORTANT NOTE: If you have 2 or more walls that come together at an
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edge or corner (e.g. walls in the x and y dimensions), then be sure to
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set the <I>overlap</I> keyword to <I>yes</I> in the <A HREF = "fix_srd.html">fix srd</A>
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command, since the walls effectively overlap when SRD particles
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collide with them. LAMMPS will issue a warning if you do not do this.
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</P>
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<P>IMPORTANT NOTE: The walls of this fix only interact with SRD
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particles, as defined by the <A HREF = "fix_srd.html">fix srd</A> command. If you
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are simulating a mixture containing other kinds of particles, then you
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should typically use <A HREF = "fix_wall.html">another wall command</A> to act on
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the other particles. Since SRD particles will be colliding both with
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the walls and the other particles, it is important to insure that the
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other particle's finite extent does not overlap an SRD wall. If you
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do not do this, you may generate errors when SRD particles end up
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"inside" another particle or a wall at the beginning of a collision
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step.
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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 a wall position, but only when a numeric constant is used.
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It is not relevant when EDGE or a variable is used to specify a face
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position.
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</P>
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<P>A <I>box</I> value selects standard distance units as defined by the
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<A HREF = "units.html">units</A> command, e.g. Angstroms for units = real or metal.
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A <I>lattice</I> value means the distance units are in lattice spacings.
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The <A HREF = "lattice.html">lattice</A> command must have been previously used to
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define the lattice spacings.
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</P>
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<HR>
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<P>Here are examples of variable definitions that move the wall position
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in a time-dependent fashion using equal-style
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<A HREF = "variable.html">variables</A>.
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</P>
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<PRE>variable ramp equal ramp(0,10)
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fix 1 all wall xlo v_ramp 1.0 1.0 2.5
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</PRE>
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<PRE>variable linear equal vlinear(0,20)
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fix 1 all wall xlo v_linear 1.0 1.0 2.5
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</PRE>
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<PRE>variable wiggle equal swiggle(0.0,5.0,3.0)
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fix 1 all wall xlo v_wiggle 1.0 1.0 2.5
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</PRE>
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<PRE>variable wiggle equal cwiggle(0.0,5.0,3.0)
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fix 1 all wall xlo v_wiggle 1.0 1.0 2.5
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</PRE>
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<P>The ramp(lo,hi) function adjusts the wall position linearly from lo to
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hi over the course of a run. The linear(c0,velocity) function does
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something similar using the equation position = c0 + velocity*delta,
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where delta is the elapsed time.
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</P>
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<P>The swiggle(c0,A,period) function causes the wall position to
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oscillate sinusoidally according to this equation, where omega = 2 PI
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/ period:
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</P>
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<PRE>position = c0 + A sin(omega*delta)
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</PRE>
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<P>The cwiggle(c0,A,period) function causes the wall position to
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oscillate sinusoidally according to this equation, which will have an
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initial wall velocity of 0.0, and thus may impose a gentler
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perturbation on the particles:
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</P>
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<PRE>position = c0 + A (1 - cos(omega*delta))
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</PRE>
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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>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
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are relevant to this fix.
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</P>
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<P>This fix computes a global array of values which can be accessed by
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various <A HREF = "Section_howto.html#howto_15">output commands</A>. The number of
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rows in the array is equal to the number of walls defined by the fix.
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The number of columns is 3, for the x,y,z components of force on each
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wall.
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</P>
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<P>Note that an outward normal force on a wall will be a negative value
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for <I>lo</I> walls and a positive value for <I>hi</I> walls. The array values
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calculated by this fix are "extensive".
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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><B>Restrictions:</B>
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</P>
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<P>Any dimension (xyz) that has an SRD wall must be non-periodic.
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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_srd.html">fix srd</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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