forked from lijiext/lammps
196 lines
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HTML
196 lines
9.9 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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<HR>
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<H3>variable command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>variable name style args ...
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</PRE>
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<UL><LI>name = name of variable to define
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<LI>style = <I>index</I> or <I>loop</I> or <I>equal</I> or <I>world</I> or <I>universe</I>
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<PRE> <I>index</I> args = one or more strings
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<I>loop</I> args = N = integer size of loop
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<I>equal</I> args = one string containing functions, vectors, keywords, numbers
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math functions = add(x,y), sub(x,y), mult(x,y), div(x,y),
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neg(x), pow(x,y), exp(x), ln(x), sqrt(x)
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group functions = mass(group), charge(group), xcm(group,dim),
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vcm(group,dim), bound(group,xmin), gyration(group)
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vectors = x[5], y[12], z[17], vx[88], vy[19], vz[2],
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fx[1], fy[2005], fz[1]
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keywords = same keywords (mostly) as in <A HREF = "thermo_style.html">thermo_style custom</A> command
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<I>world</I> args = one string for each partition of processors
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<I>universe</I> args = one or more strings
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<I>uloop</I> args = N = integer size of loop
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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>variable x index run1 run2 run3 run4 run5 run6 run7 run8
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variable LoopVar loop 20
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variable beta equal div(temp,3.0)
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variable b1 equal add(x[234],mult(0.5,lx))
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variable b equal xcm(mol1,x)
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variable temp world 300.0 310.0 320.0 330.0
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variable x universe 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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variable x uloop 15
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>This command assigns one or more values to a variable name so that the
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variable can be used in subsequent input script commands. The "name"
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of the variable is an arbitrary string. Each "value" is a string
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which could be text or numbers, as in the examples above. As
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explained in <A HREF = "Section_commands.html#3_2">this section</A>, occurrences of
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the variable name in an input script line are replaced by the
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variable's value. The variable name can be referenced in the input
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script as $x if the name "x" is a single character, or as ${LoopVar}
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if the name "LoopVar" is one or more characters.
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</P>
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<P>As described below, for variable styles <I>index</I>, <I>loop</I>, <I>universe</I>,
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and <I>uloop</I>, the value assigned to a variable can be incremented via
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the <A HREF = "next.html">next</A> command. When there are no more values to
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assign, the variable is "exhausted" and a flag is set that causes the
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next <A HREF = "jump.html">jump</A> command encountered in the input script to be
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skipped. This enables the construction of simple loops in the input
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script that are iterated over and exited from.
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</P>
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<P>When a variable command is encountered for a variable that has already
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been specified, the command is ignored. This allows an input script
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with a variable command to be processed multiple times; see the
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<A HREF = "jump.html">jump</A> or <A HREF = "include.html">include</A> commands. It also means
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that the use of the command-line switch -var will override a
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corresponding variable setting in the input script.
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</P>
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<P>There are two exceptions to this rule. The first is for <I>equal</I> style
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variables. They are re-defined each time a variable command using an
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equal-style variable is encountered. Also, if a variable is iterated
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on to the end of its list via the <A HREF = "next.html">next</A> command, it is
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available to be re-defined in a subsequent variable command.
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</P>
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<P>For the <I>index</I> style, one or more strings are specified. Initially,
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the 1st string is assigned to the variable. Each time a
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<A HREF = "next.html">next</A> command is used with the variable name, the next
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string is assigned. All processors assign the same string to the
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variable. <I>Index</I>-style variables can also be set (with a single
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value) by using the command-line switch -var; see <A HREF = "Section_start.html#2_4">this
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section</A> for details.
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</P>
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<P>The <I>loop</I> style is identical to the <I>index</I> style except that the
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strings are the integers from 1 to N. This allows you to generate a
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long list of runs (e.g. 1000) without having to list N values in your
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input script. Initially, the string "1" is assigned to the variable.
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Each time a <A HREF = "next.html">next</A> command is used with the variable name,
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the next string ("2", "3", etc) is assigned. All processors assign
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the same string to the variable.
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</P>
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<P>For the <I>equal</I> style, a single string is specified which represents
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an equation that will be evaluated afresh each time the variable is
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used. Thus the variable can take on different values at different
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stages of the input script. For example, if the variable is used in a
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<A HREF = "fix_print.html">fix print</A> command, it could print different values
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each timestep it was invoked. The next command cannot be used with
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<I>equal</I>-style variables, since there is only one value. Note that, as
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with any other input script command, it is feasible to use another
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variable in the <I>equal</I> variable's string, e.g. variable y equal
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mult($x,2). However, $x will be replaced immediately by it's current
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value when the command is first parsed, not each time that $y is
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substituted for.
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</P>
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<P>The syntax of the equation assigned to <I>equal</I> variables is simple.
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It can contain "functions", "vectors", "keywords", or "numbers" in any
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combination.
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</P>
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<UL><LI>Function = a keyword followed by parenthesis with one or two arguments
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<LI>Supported arithmetic functions = add(x,y), sub(x,y), mult(x,y), div(x,y), neg(x), pow(x,y), exp(x), ln(x), sqrt(x)
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<LI>Supported group functions = mass(ID), charge(ID), xcm(ID,dim), vcm(ID,dim), bound(ID,dir), gyration(ID)
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<LI>Example function usage = div(1.0e20,3.0), neg(x[34]), pow(lx,3.0), xcm(mol,x), bound(lower,zmin)
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<LI>Vector = a keyword followed by square brackets containing an atom ID
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<LI>Supported vectors = x, y, z, vx, vy, vz, fx, fy, fz
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<LI>Example vector usage = x[123], fz[1000]
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<LI>Keyword = any keyword supported by the <A HREF = "thermo_style.html">thermo_style custom</A> command except cpu, pressure tensor components (pxx, pyy, etc), time-averaged quantities (tave, pave, etc)
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<LI>Example keyword usage = atoms, pow(vol,0.333), mult(elong,0.5)
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<LI>Number = 0.2, 1.0e20, -15.4, etc
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</UL>
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<P>For the group functions, ID is a group-ID, dim is 'x' or 'y' or 'z',
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and dir is one of 6 strings: "xmin", "xmax", "ymin", "ymax", "zmin",
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or "zmax". The group functions mass() and charge() are the total mass
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and charge of the group of atoms. Xcm() and vcm() return components
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of the position and velocity of the center of mass of the group.
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Bound() returns the min/max of a particular coordinate for all atoms
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in the group. Gyration() computes the radius-of-gyration of the group
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of atoms. See the <A HREF = "fix_gyration.html">fix gyration</A> command for the
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formula.
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</P>
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<P>Keywords have restrictions on when they can be assigned to variables.
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For example, keywords that compute thermodynamic quantites can only be
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invoked after the first simulation has begun. A warning is issued if
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thermodyanmic keywords are invoked on timesteps when thermodynamic
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information is not being printed to the screen, since the values
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assigned to the variable may be out-of-date.
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</P>
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<P>The variable <I>equal</I> equation can also be nested in that function
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arguments can be functions, vectors, keywords, or numbers. For
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example, this is a valid equation:
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</P>
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<PRE>variable x equal div(add(pe,ke),pow(vol,div(1,3)))
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</PRE>
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<P>For the <I>world</I> style, one or more strings are specified. There must
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be one string for each processor partition or "world". See <A HREF = "Section_start.html#2_4">this
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section</A> of the manual for information on
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running LAMMPS with multiple partitions via the "-partition"
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command-line switch. This variable command assigns one string to each
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world. All processors in the world are assigned the same string. The
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next command cannot be used with <I>equal</I>-style variables, since there
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is only one value per world. This style of variable is useful when
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you wish to run different simulations on different partitions, or when
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performing a parallel tempering simulation (see the
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<A HREF = "temper.html">temper</A> command), to assign different temperatures to
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different partitions.
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</P>
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<P>For the <I>universe</I> style, one or more strings are specified. There
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must be at least as many strings as there are processor partitions or
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"worlds". See <A HREF = "Section_start.html#2_4">this page</A> for information on
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running LAMMPS with multiple partitions via the "-partition"
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command-line switch. This variable command initially assigns one
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string to each world. When a <A HREF = "next.html">next</A> command is encountered
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using this variable, the first processor partition to encounter it, is
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assigned the next available value. This continues until all the
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variable values are consumed. Thus, this command can be used to run
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50 simulations on 8 processor partitions. The simulations will be run
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one after the other on whatever partition becomes available, until
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they are all finished. <I>Universe</I>-style variables are incremented
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using the files "tmp.lammps.variable" and "tmp.lammps.variable.lock"
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which you will see in your directory during such a LAMMPS run.
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</P>
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<P>The <I>uloop</I> style is identical to the <I>universe</I> style except that the
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strings are the integers from 1 to N. This allows you to generate a
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long list of runs (e.g. 1000) without having to list N values in your
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input script.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>The use of atom vectors in <I>equal</I> style variables requires the atom
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style to use a global mapping in order to look up the vector indices.
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Only atom styles with molecular information create global maps.
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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 = "next.html">next</A>, <A HREF = "jump.html">jump</A>, <A HREF = "include.html">include</A>,
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<A HREF = "temper.html">temper</A>, <A HREF = "fix_print.html">fix print</A>, <A HREF = "print.html">print</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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