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576 lines
28 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>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>delete</I> or <I>index</I> or <I>loop</I> or <I>world</I> or <I>universe</I> or <I>uloop</I> or <I>equal</I> or <I>atom</I>
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<PRE> <I>delete</I> = no args
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<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>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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<I>equal</I> or <I>atom</I> args = one formula containing numbers, thermo keywords, math operations, group functions, atom values and vectors, compute/fix/variable references
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numbers = 0.0, 100, -5.4, 2.8e-4, etc
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thermo keywords = vol, ke, press, etc from <A HREF = "thermo_style.html">thermo_style</A>
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math operations = (), -x, x+y, x-y, x*y, x/y, x^y,
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sqrt(x), exp(x), ln(x), log(x),
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sin(x), cos(x), tan(x), asin(x), acos(x), atan(x),
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ceil(x), floor(x), round(x)
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group functions = count(group), mass(group), charge(group),
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xcm(group,dim), vcm(group,dim), fcm(group,dim),
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bound(group,xmin), gyration(group)
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atom value = mass[N], x[N], y[N], z[N],
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vx[N], vy[N], vz[N], fx[N], fy[N], fz[N]
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atom vector = mass[], x[], y[], z[],
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vx[], vy[], vz[], fx[], fy[], fz[]
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compute references = c_ID, c_ID[2], c_ID[N], c_ID[N][2], c_ID[], c_ID[][2]
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fix references = f_ID, f_ID[2], f_ID[N], f_ID[N][2], f_ID[], f_ID[][2]
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variable references = v_abc, v_abc[N], v_abc[]
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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 $n
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variable beta equal temp/3.0
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variable b1 equal x[234]+0.5*vol
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variable b1 equal "x[234] + 0.5*vol"
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variable b equal xcm(mol1,x)/2.0
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variable b equal c_myTemp
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variable b atom x[]*y[]/vol
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variable temp world 300.0 310.0 320.0 ${Tfinal}
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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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variable x delete
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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 strings to a variable name for
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evaluation later in the input script or during a simulation.
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</P>
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<P>Variables can be used in several ways in LAMMPS. A variable can be
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referenced elsewhere in an input script to become part of a new input
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command. For variable styles that store multiple strings, the
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<A HREF = "next.html">next</A> command can be used to increment which string is
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assigned to the variable. Variables of style <I>equal</I> store a formula
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which when evaluated produces a single numeric value which can be
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output either directly (see the <A HREF = "print.html">print</A>, <A HREF = "fix_print.html">fix
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print</A>, and <A HREF = "run.html">run every</A> commands) or as part
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of thermodynamic output (see the <A HREF = "thermo_style.html">thermo_style</A>
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command), or used as input to an averaging fix (see the <A HREF = "fix_ave/time">fix
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ave/time</A> command). Variables of style <I>atom</I> store a
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formula which when evaluated produces one numeric value per atom which
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can be output to a dump file (see the <A HREF = "dump.html">dump custom</A> command)
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or used as input to an averaging fix (see the <A HREF = "fix_ave_spatial.html">fix
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ave/spatial</A> and <A HREF = "fix_ave_atom.html">fix ave/atom</A>
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commands).
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</P>
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<P>In the discussion that follows, the "name" of the variable is the
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arbitrary string that is the 1st argument in the variable command.
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This name can only contain alphanumeric characters and underscores.
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The "string" is one or more of the subsequent arguments. The "string"
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can be simple text as in the 1st example above, it can contain other
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variables as in the 2nd example, or it can be a formula as in the 3rd
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example. The "value" is the numeric quantity resulting from
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evaluation of the string. Note that the same string can generate
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different values when it is evaluated at different times during a
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simulation.
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</P>
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<P>IMPORTANT NOTE: When the input script line that defines a variable of
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style <I>equal</I> or <I>atom</I> that contain a formula is encountered, the
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formula is NOT immediately evaluated and the result stored. See the
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discussion below about "Immediate Evaluation of Variables" if you want
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to do this.
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</P>
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<P>IMPORTANT NOTE: When a variable command is encountered in the input
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script and the variable name has already been specified, the command
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is ignored. This means variables can NOT be re-defined in an input
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script (with 2 exceptions, read further). This is to allow an input
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script to be processed multiple times without resetting the variables;
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see the <A HREF = "jump.html">jump</A> or <A HREF = "include.html">include</A> commands. It also
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means that using the <A HREF = "Section_start.html#2_6">command-line switch</A> -var
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will override a corresponding index variable setting in the input
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script.
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</P>
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<P>There are two exceptions to this rule. First, variables of style
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<I>equal</I> and <I>atom</I> ARE redefined each time the command is encountered.
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This only changes their associated formula if the formula contains a
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substitution for another variable, e.g. $x. But that can be useful,
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for example, in a loop.
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</P>
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<P>Second, as described below, if a variable is iterated on to the end of
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its list of strings via the <A HREF = "next.html">next</A> command, it is removed
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from the list of active variables, and is thus available to be
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re-defined in a subsequent variable command. The <I>delete</I> option does
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the same thing.
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</P>
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<HR>
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<P><A HREF = "Section_commands.html#3_2">This section</A> of the manual explains how
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occurrences of a variable name in an input script line are replaced by
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the variable's string. The variable name can be referenced as $x if
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the name "x" is a single character, or as ${LoopVar} if the name
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"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>, which string is assigned to a variable can be incremented
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via the <A HREF = "next.html">next</A> command. When there are no more strings 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 then exited from.
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</P>
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<P>As explained above, an exhausted variable can be re-used in an input
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script. The <I>delete</I> option also removes the variable, the same as if
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it were exhausted, allowing it to be redefined later in the input
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script or when the input script is looped over. This can be useful
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when breaking out of a loop via the <A HREF = "if.html">if</A> and <A HREF = "jump.html">jump</A>
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commands before the variable would become exhausted. For example,
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</P>
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<PRE>label loop
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variable a loop 5
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print "A = $a"
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if $a > 2 then "jump in.script break"
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next a
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jump in.script loop
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label break
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variable a delete
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</PRE>
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<HR>
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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.
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</P>
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<P><I>Index</I> style variables with a single string value can also be set by
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using the command-line switch -var; see <A HREF = "Section_start.html#2_6">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 generation of a
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long list of runs (e.g. 1000) without having to list N strings in the
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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>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_6">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_6">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 string. This continues until all the
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variable strings 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 generation of long
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list of runs (e.g. 1000) without having to list N strings in the input
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script.
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</P>
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<HR>
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<P>For the <I>equal</I> and <I>atom</I> styles, a single string is specified which
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represents a formula that will be evaluated afresh each time the
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variable is used. If you want spaces in the string, enclose it in
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double quotes so the parser will treat it as a single argument. For
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<I>equal</I> style variables the formula computes a scalar quantity, which
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becomes the value of the variable whenever it is evaluated. For
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<I>atom</I> style variables the formula computes one quantity for each
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atom whenever it is evaluated.
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</P>
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<P>Note that <I>equal</I> and <I>atom</I> variables can produce different values at
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different stages of the input script or at different times during a
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run. For example, if an <I>equal</I> variable is used in a <A HREF = "fix_print.html">fix
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print</A> command, different values could be printed each
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timestep it was invoked. If you want a variable to be evaluated
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immediately, so that the result is stored by the variable instead of
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the string, see the section below on "Immediate Evaluation of
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Variables".
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</P>
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<P>The next command cannot be used with <I>equal</I> or <I>atom</I> style
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variables, since there is only one string.
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</P>
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<P>The formula for an <I>equal</I> or <I>atom</I> variable can contain a variety
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of quantities. The syntax for each kind of quantity is simple, but
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multiple quantities can be nested and combined in various ways to
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build up formulas of arbitrary complexity. For example, this is a
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valid (though strange) variable formula:
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</P>
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<PRE>variable x equal "pe + c_MyTemp / vol^(1/3)"
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</PRE>
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<P>Specifically, an formula can contain numbers, thermo keywords, math
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operations, group functions, atom values, atom vectors, compute
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references, fix references, and references to other variables.
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</P>
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<DIV ALIGN=center><TABLE BORDER=1 >
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<TR><TD >Number</TD><TD > 0.2, 100, 1.0e20, -15.4, etc</TD></TR>
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<TR><TD >Thermo keywords</TD><TD > vol, pe, ebond, etc</TD></TR>
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<TR><TD >Math operations</TD><TD > (), -x, x+y, x-y, x*y, x/y, x^y, sqrt(x), exp(x), ln(x), log(x), sin(x), cos(x), tan(x), asin(x), acos(x), atan(x), ceil(x), floor(x), round(x)</TD></TR>
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<TR><TD >Group functions</TD><TD > count(ID), mass(ID), charge(ID), xcm(ID,dim), vcm(ID,dim), fcm(ID,dim), bound(ID,dir), gyration(ID)</TD></TR>
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<TR><TD >Atom values</TD><TD > mass[N], x[N], y[N], z[N], vx[N], vy[N], vz[N], fx[N], fy[N], fz[N]</TD></TR>
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<TR><TD >Atom vectors</TD><TD > mass[], x[], y[], z[], vx[], vy[], vz[], fx[], fy[], fz[]</TD></TR>
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<TR><TD >Compute references</TD><TD > c_ID, c_ID[2], c_ID[N], c_ID[N][2], c_ID[], c_ID[][2]</TD></TR>
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<TR><TD >Fix references</TD><TD > f_ID, f_ID[2], f_ID[N], f_ID[N][2], f_ID[], f_ID[][2]</TD></TR>
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<TR><TD >Other variables</TD><TD > v_abc, v_abc[N], v_abc[]
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</TD></TR></TABLE></DIV>
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<P>Note that formula elements that contain empty brackets, such as an
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atom vector, produce per-atom values. All other formula elements
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produce a global value.
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</P>
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<P>A formula for equal-style variables cannot use any formula element
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that produces per-atom values. A formula for an atom-style variable
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can use formula elements that produce either global values or per-atom
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values.
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</P>
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<P>The thermo keywords allowed in a formula are those defined by the
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"thermo_style custom" command. Thermo keywords that require a
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<A HREF = "compute.html">compute</A> to calculate their values such as "temp" or
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"press", use computes stored and invoked by the thermo_style command.
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This means that you can only use those keywords in a variable if the
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style you are using with the thermo_style command (and the thermo
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keywords associated with that style) also define and use the needed
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compute. Note that some thermo keywords use a compute indirectly to
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calculate their value (e.g. the enthalpy keyword uses temp, pe, and
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pressure). If a variable is evaluated in an input script (not during
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a run), then the values accessed by the thermo keyword must be
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current. See the discussion below about "Variable Accuracy".
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</P>
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<P>Math operations are written in the usual way, where the "x" and "y" in
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the examples above can be another section of the formula. Operators
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are evaluated left to right and have the usual precedence: unary minus
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before exponentiation ("^"), exponentiation before multiplication and
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division, and multiplication and division before addition and
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subtraction. Parenthesis can be used to group one or more portions of
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a formula and enforce a desired order of operations. Additional math
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operations can be specified as keywords followed by a parenthesized
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argument, e.g. sqrt(v_ke). Note that ln() is the natural log; log()
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is the base 10 log. The ceil(), floor(), and round() operations are
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those in the C math library. Ceil() is the smallest integer not less
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than its argument. Floor() if the largest integer not greater than
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its argument. Round() is the nearest integer to its argument.
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</P>
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<P>Group functions take one or two arguments in a specific format. The
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first argument is the group-ID. The <I>dim</I> argument, if it exists, is
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<I>x</I> or <I>y</I> or <I>z</I>. The <I>dir</I> argument, if it exists, is <I>xmin</I>,
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<I>xmax</I>, <I>ymin</I>, <I>ymax</I>, <I>zmin</I>, or <I>zmax</I>. The group function count()
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is the number of atoms in the group. The group functions mass() and
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charge() are the total mass and charge of the group. Xcm() and vcm()
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return components of the position and velocity of the center of mass
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of the group. Fcm() returns a component of the total force on the
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group of atoms. Bound() returns the min/max of a particular
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coordinate for all atoms in the group. Gyration() computes the
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radius-of-gyration of the group of atoms. See the <A HREF = "fix_gyration.html">fix
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gyration</A> command for a definition of the formula.
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</P>
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<P>Atom values take a single integer argument from 1-N, which is the
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desired atom-ID, e.g. x[243]., which means use the x coordinate of
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the atom with ID=243.
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</P>
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<P>Atom vectors use empty brackets, i.e. they take no argument. They
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generate one value per atom, so that a reference like x[] means the
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x-coord of each atom will be used when evaluating the variable.
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</P>
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<P>Compute references access one or more quantities calculated by a
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<A HREF = "compute.html">compute</A>. The ID in the reference should be replaced by
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the actual ID of the compute defined elsewhere in the input script.
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See the doc pages for individual computes to see which ones calculate
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global versus per-atom quantities. If the compute reference contains
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empty brackets, then per-atom values calculated by the compute are
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accessed. Otherwise a single value (global or per-atom) calculated by
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the compute is accessed. If a variable containing a compute is
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evaluated in an input script (not during a run), then the values
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accessed by the compute must be current. See the discussion below
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about "Variable Accuracy".
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</P>
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<P>The different kinds of compute references are as follows. M is a
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positive integer <= the number of vector values calculated by the
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compute. N is a global atom ID (positive integer).
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</P>
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<DIV ALIGN=center><TABLE BORDER=1 >
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<TR><TD >c_ID</TD><TD > scalar value of a global compute</TD></TR>
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<TR><TD >c_ID[2]</TD><TD > vector component of a global compute</TD></TR>
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<TR><TD >c_ID[N]</TD><TD > single atom's scalar value of a per-atom compute</TD></TR>
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<TR><TD >c_ID[N][M]</TD><TD > single atom's vector component of a per-atom compute</TD></TR>
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<TR><TD >c_ID[]</TD><TD > per-atom scalar from a per-atom compute</TD></TR>
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<TR><TD >c_ID[][M]</TD><TD > per-atom vector component from a per-atom compute
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</TD></TR></TABLE></DIV>
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<P>Fix references access one or more quantities calculated by a
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<A HREF = "fix.html">fix</A>. The ID in the reference should be replaced by the
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actual ID of the fix defined elsewhere in the input script. See the
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doc pages for individual computes to see which ones calculate global
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versus per-atom quantities. If the fix reference contains empty
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brackets, then per-atom values calculated by the fix are accessed.
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Otherwise a single value (global or per-atom) calculated by the fix is
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accessed.
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</P>
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<P>Note that some fixes only generate quantities on certain timesteps.
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If a variable attempts to access the fix on non-allowed timesteps, an
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error is generated. For example, the <A HREF = "fix_ave_time.html">fix ave/time</A>
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command may only generate averaged quantities every 100 steps. See
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the doc pages for individual fix commands for details. If a variable
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containing a fix is evaluated in an input script (not during a run),
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then the values accessed by the fix should be current. See the
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discussion below about "Variable Accuracy".
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</P>
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<P>The different kinds of fix references are exactly the same as the
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compute references listed in the above table, where "c_" is replaced
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by "f_", and the word "compute" is replaced by "fix".
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</P>
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<P>The current values of other variables can be accessed by prepending a
|
|
"v_" to the variable name. This will cause that variable to be
|
|
evaluated. Atom-style variables generate per-atom values; all other
|
|
styles of variables generate a single scalar value.
|
|
</P>
|
|
<P>The different kinds of variable references are as follows. N is a
|
|
global atom ID (positive integer).
|
|
</P>
|
|
<DIV ALIGN=center><TABLE BORDER=1 >
|
|
<TR><TD >v_ID</TD><TD > scalar value of a non atom-style variable</TD></TR>
|
|
<TR><TD >v_ID[N]</TD><TD > single atom's scalar value from an atom-style variable</TD></TR>
|
|
<TR><TD >v_ID[]</TD><TD > per-atom value from an atom-style variable
|
|
</TD></TR></TABLE></DIV>
|
|
|
|
<P>IMPORTANT NOTE: If you define variables in circular manner like this:
|
|
</P>
|
|
<PRE>variable a equal v_b
|
|
variable b equal v_a
|
|
print $a
|
|
</PRE>
|
|
<P>then LAMMPS will run for a while when the print statement is invoked!
|
|
</P>
|
|
<HR>
|
|
|
|
<P><B>Immediate Evaluation of Variables:</B>
|
|
</P>
|
|
<P>There is a difference between referencing a variable with a leading $
|
|
sign (e.g. $x or ${abc}) versus with a leading "v_" (e.g. v_x or
|
|
v_abc). The former can be used in any command, including a variable
|
|
command, to force the immediate evaluation of the referenced variable
|
|
and the substitution of its value into the command. The latter is a
|
|
required kind of argument to some commands (e.g. the <A HREF = "fix_ave_spatial.html">fix
|
|
ave/spatial</A> or <A HREF = "dump.html">dump custom</A> or
|
|
<A HREF = "thermo_style.html">thermo_style</A> commands) if you wish it to evaluate
|
|
a variable periodically during a run. It can also be used in a
|
|
variable formula if you wish to reference a second variable. The
|
|
second variable will be evaluated whenever the first variable is
|
|
evaluated.
|
|
</P>
|
|
<P>As an example, suppose you use this command in your input script to
|
|
define the variable "v" as
|
|
</P>
|
|
<PRE>variable v equal vol
|
|
</PRE>
|
|
<P>before a run where the simulation box size changes. You might think
|
|
this will assign the initial volume to the variable "v". That is not
|
|
the case. Rather it assigns a formula which evaluates the volume
|
|
(using the thermo_style keyword "vol") to the variable "v". If you
|
|
use the variable "v" in some other command like "fix ave/time" then
|
|
the current volume of the box will be evaluated continuously during
|
|
the run.
|
|
</P>
|
|
<P>If you want to store the initial volume of the system, you can do it
|
|
this way:
|
|
</P>
|
|
<PRE>variable v equal vol
|
|
variable v0 equal $v
|
|
</PRE>
|
|
<P>The second command will force "v" to be evaluated (yielding the
|
|
initial volume) and assign that value to the variable "v0". Thus the
|
|
command
|
|
</P>
|
|
<PRE>thermo_style custom step v_v v_v0
|
|
</PRE>
|
|
<P>would print out both the current and initial volume periodically
|
|
during the run.
|
|
</P>
|
|
<P>Note that it is a mistake to enclose a variable formula in double
|
|
quotes if it contains variables preceeded by $ signs. For example,
|
|
</P>
|
|
<PRE>variable vratio equal "${vfinal}/${v0}"
|
|
</PRE>
|
|
<P>This is because the quotes prevent variable substitution (see <A HREF = "Section_commands.html#3_2">this
|
|
section</A> on parsing input script commands),
|
|
and thus an error will occur when the formula for "vratio" is
|
|
evaluated later.
|
|
</P>
|
|
<HR>
|
|
|
|
<P><B>Variable Accuracy:</B>
|
|
</P>
|
|
<P>Obviously, LAMMPS attempts to evaluate variables containing formulas
|
|
(<I>equal</I> and <I>atom</I> style variables) accurately whenever the
|
|
evaluation is performed. Depending on what is included in the
|
|
formula, this may require invoking a <A HREF = "compute.html">compute</A>, either
|
|
directly or indirectly via a thermo keyword, or accessing a value
|
|
calculated and stored by a <A HREF = "fix.html">fix</A>. If the compute is one that
|
|
calculates the pressure or energy of the system, then these quantities
|
|
need to be tallied during the evaluation of the interatomic potentials
|
|
(pair, bond, etc) on timesteps that the variable will need the values.
|
|
</P>
|
|
<P>LAMMPS keeps track of all of this during a <A HREF = "run.html">run</A> or <A HREF = "minimize.html">energy
|
|
minimization</A>. An error will be generated if you
|
|
attempt to evaluate a variable on timesteps when it cannot produce
|
|
accurate values. For example, if a <A HREF = "thermo_style.html">thermo_style
|
|
custom</A> command prints a variable which accesses
|
|
values stored by a <A HREF = "fix_ave_time.html">fix ave/time</A> command and the
|
|
timesteps on which thermo output is generated are not multiples of the
|
|
averaging frequency used in the fix command, then it is an error.
|
|
</P>
|
|
<P>However, your input script can also require variables to be evaluated
|
|
before or after or inbetween runs. In this case, one of three things
|
|
may happen.
|
|
</P>
|
|
<P>(1) The variable may be evaluated accurately. If it contains
|
|
references to a compute or fix, and these values were calculated and
|
|
stored on the last timestep of a preceeding run, then they can be
|
|
accessed and used by the variable.
|
|
</P>
|
|
<P>(2) LAMMPS may not be able to evaluate the variable and generate an
|
|
error. For example, if the variable formula requires a
|
|
<A HREF = "compute.html">compute</A> to be invoked, either directly or indirectly
|
|
via a thermo keyword, then the variable cannot be evaluated before the
|
|
first run has occurred. The general rule is that if a variable uses a
|
|
value calculated by a compute, and the variable is not being evaluated
|
|
during a run, then the variable will not invoke the compute. Instead,
|
|
it can only use the value stored by the compute, which for accuracy
|
|
requires that the compute was already invoked on the same timestep
|
|
during a preceeding run.
|
|
</P>
|
|
<P>Thus the way to get around this error is to perform a 0-timestep run
|
|
before using the variable. For example, these commands
|
|
</P>
|
|
<PRE>variable t equal temp
|
|
print "Initial temperature = $t"
|
|
run 1000
|
|
</PRE>
|
|
<P>will generate an error if the run is the first run specified in the
|
|
input script, because generating a value for the "t" variable requires
|
|
a compute for calculating the temperature to be invoked.
|
|
</P>
|
|
<P>However, this sequence of commands would be fine:
|
|
</P>
|
|
<PRE>run 0
|
|
variable t equal temp
|
|
print "Initial temperature = $t"
|
|
run 1000
|
|
</PRE>
|
|
<P>The 0-timestep run initializes and invokes various computes, including
|
|
the one for temperature, so that the value it stores is current and
|
|
can be accessed by the variable "t" after the run has completed. Note
|
|
that a 0-timestep run does not alter the state of the system, so it
|
|
does not change the input state for the 1000-timestep run that
|
|
follows. Also note, that the 0-timestep run must actually use and
|
|
invoke the compute in question (e.g. via <A HREF = "thermo_style.html">thermo</A> or
|
|
<A HREF = "dump.html">dump</A> output) in order for it to enable the compute to be
|
|
used in a variable after the run.
|
|
</P>
|
|
<P>Note that unlike computes, <A HREF = "fix.html">fixes</A> will never generate an
|
|
error if their values are accessed by a variable in between runs.
|
|
They always return some value to the variable. However, this value
|
|
may not be what you expect if the fix has not yet calculated the
|
|
quantity of interest. For example, the <A HREF = "fix_indent.html">fix indent</A>
|
|
command stores the force on the indenter. But this is not computed
|
|
until a run is performed. Thus if a variable attempts to print this
|
|
value before the first run, values of zero will be output. Again,
|
|
performing a 0-timestep run before printing the variable has the
|
|
desired effect.
|
|
</P>
|
|
<P>(3) The variable may be evaluated incorrectly. And LAMMPS may have
|
|
no way to detect this has occurred. Consider the following sequence
|
|
of commands:
|
|
</P>
|
|
<PRE>pair_coeff 1 1 1.0 1.0
|
|
run 1000
|
|
pair_coeff 1 1 1.5 1.0
|
|
variable e equal pe
|
|
print "Final potential energy = $e"
|
|
</PRE>
|
|
<P>The first run is performed using one setting for the pairwise
|
|
potential defined by the <A HREF = "pair_style.html">pair_style</A> and
|
|
<A HREF = "pair_coeff.html">pair_coeff</A> commands. The potential energy is
|
|
evaluated on the final timestep and stored by the <A HREF = "compute_pe.html">compute
|
|
pe</A> compute (this is done by the
|
|
<A HREF = "thermo_style.html">thermo_style</A> command). Then a pair coefficient is
|
|
changed, altering the potential energy of the system. When the
|
|
potential energy is printed via the "e" variable, LAMMPS will use the
|
|
potential energy value stored by the <A HREF = "compute_pe.html">compute pe</A>
|
|
compute, thinking it is current. There are many other commands which
|
|
could alter the state of the system between runs, causing a variable
|
|
to evaluate incorrectly.
|
|
</P>
|
|
<P>The solution to this issue is the same as for case (2) above, namely
|
|
to perform a 0-timestep run before the variable is evaluated to insure
|
|
the system is up-to-date. For example, this sequence of commands
|
|
would print a potential energy that reflected the changed pairwise
|
|
coefficient:
|
|
</P>
|
|
<PRE>pair_coeff 1 1 1.0 1.0
|
|
run 1000
|
|
pair_coeff 1 1 1.5 1.0
|
|
run 0
|
|
variable e equal pe
|
|
print "Final potential energy = $e"
|
|
</PRE>
|
|
<HR>
|
|
|
|
<P><B>Restrictions:</B>
|
|
</P>
|
|
<P>Indexing any formula element by global atom ID, such as an atom value,
|
|
requires the atom style to use a global mapping in order to look up
|
|
the vector indices. By default, only atom styles with molecular
|
|
information create global maps. The <A HREF = "atom_modify.html">atom_modify
|
|
map</A> command can override the default.
|
|
</P>
|
|
<P>All <I>universe</I>- and <I>uloop</I>-style variables defined in an input script
|
|
must have the same number of values.
|
|
</P>
|
|
<P><B>Related commands:</B>
|
|
</P>
|
|
<P><A HREF = "next.html">next</A>, <A HREF = "jump.html">jump</A>, <A HREF = "include.html">include</A>,
|
|
<A HREF = "temper.html">temper</A>, <A HREF = "fix_print.html">fix print</A>, <A HREF = "print.html">print</A>
|
|
</P>
|
|
<P><B>Default:</B> none
|
|
</P>
|
|
</HTML>
|