forked from lijiext/lammps
296 lines
13 KiB
Plaintext
296 lines
13 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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variable command :h3
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[Syntax:]
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variable name style args ... :pre
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name = name of variable to define :ulb,l
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style = {index} or {loop} or {world} or {universe} or {uloop} or {equal} or {atom} :l
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{index} args = one or more strings
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{loop} args = N = integer size of loop
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{world} args = one string for each partition of processors
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{universe} args = one or more strings
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{uloop} args = N = integer size of loop
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{equal} or {atom} args = one equation containing numbers, thermo keywords, math functions, group functions, atom vectors, compute references, other variables
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numbers = 0.0, -5.4, 2.8e-4, etc
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thermo keywords = vol, ke, press, etc from "thermo_style"_thermo_style.html
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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),
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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 vectors for {equal} = mass\[N\], x\[N\], y\[N\], z\[N\],
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vx\[N\], vy\[N\], vz\[N\],
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fx\[N\], fy\[N\], fz\[N\]
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atom vectors for {atom} = mass\[\], x\[\], y\[\], z\[\],
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vx\[\], vy\[\], vz\[\],
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fx\[\], fy\[\], fz\[\]
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compute references = c_ID\[0\], c_ID\[N\]
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other variables = v_abc, v_x, etc :pre
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:ule
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[Examples:]
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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,vol))
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variable b equal div(xcm(mol1,x),2.0)
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variable b equal c_myTemp\[0\]
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variable b atom div(mult(x[],y[]),vol)
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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 :pre
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[Description:]
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This command assigns one or more strings to a variable name.
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Variables can be used in several ways in LAMMPS. A variable can be
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referenced elsewhere in an input script. For variable styles that
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store multiple strings, the "next"_next.html command can be used to
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increment which string is assigned to the variable. Variables can be
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evaluated to produce a numeric value which can be output either
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directly (see the "print"_print.html, "fix print"_fix_print.html, and
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"run every"_run.html commands) or as part of thermodynamic output (see
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the "thermo_style"_thermo_style.html command), fix output (see the
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"fix ave/spatial"_fix_ave_spatial.html and "compute
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variable/atom"_compute_variable_atom.html commands), or dump output
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(see the "dump custom"_dump.html and "compute
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variable/atom"_compute_variable_atom.html commands).
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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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The "string" is one of the subsequent arguments. The "value" is the
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numeric quantity resulting from evaluation of the string. Note that
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the same string can generate different values when it is evaluated at
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different times during a simulation.
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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. This is to allow an input script to be processed multiple
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times without resetting the variables; see the "jump"_jump.html or
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"include"_include.html commands. It also means that using a
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command-line switch -var will override a corresponding variable
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setting in the input script.
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There is one exception to this rule. As described below, if a
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variable is iterated on to the end of its list of strings via the
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"next"_next.html command, it is available to be re-defined in a
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subsequent variable command.
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:line
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"This section"_Section_commands.html#3_2 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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As described below, for variable styles {index}, {loop}, {universe},
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and {uloop}, the string assigned to a variable can be incremented via
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the "next"_next.html 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 "jump"_jump.html 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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:line
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For the {index} 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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"next"_next.html 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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The {loop} style is identical to the {index} 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 "next"_next.html 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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For the {world} style, one or more strings are specified. There must
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be one string for each processor partition or "world". See "this
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section"_Section_start.html#2_6 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 {equal} 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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"temper"_temper.html command), to assign different temperatures to
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different partitions.
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For the {universe} 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 "this page"_Section_start.html#2_6 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 "next"_next.html 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. {Universe} 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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The {uloop} style is identical to the {universe} 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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:line
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For the {equal} and {atom} styles, a single string is specified which
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represents an equation that will be evaluated afresh each time the
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variable is used. For {equal} style variables this equation computes
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a scalar quantity, which becomes the value of the variable whenever it
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is evaluated. For {atom} style variables it computes a quantity for
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each atom, which is used by the "compute variable/atom" command and
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its associated output.
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Note that {equal} and {atom} 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 {equal} variable is used in a "fix
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print"_fix_print.html command, different values could be printed each
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timestep it was invoked.
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{Equal} style variables can also be set by using the command-line
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switch -var; see "this section"_Section_start.html#2_6 for details.
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The next command cannot be used with {equal} or {atom} style
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variables, since there is only one string.
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The equation for an {equal} or {atom} 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 equation:
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variable x equal div(add(pe,c_MyTemp\[0\]),pow(vol,div(1,3))) :pre
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Specifically, an equation can contain numbers, thermo keywords, math
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functions, group functions, atom vectors, compute references, and
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other variables. There is one difference between {equal} and {atom}
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variables; the syntax of Atom vector references is different.
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Number: 0.2, 1.0e20, -15.4, etc
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Thermo keywords: vol, pe, ebond, etc
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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(ID), charge(ID), xcm(ID,dim), vcm(ID,dim), fcm(ID,dim) \
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bound(ID,dir), gyration(ID)
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Atom vectors for {equal}: mass\[N\], x\[N\], y\[N\], z\[N\], \
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vx\[N\], vy\[N\], vz\[N\], \
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fx\[N\], fy\[N\], fz\[N\]
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Atom vectors for {atom}: mass\[\], x\[\], y\[\], z\[\], \
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vx\[\], vy\[\], vz\[\], \
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fx\[\], fy\[\], fz\[\]
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Compute references: c_ID\[0\], c_ID\[N\]
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Other variables: v_abc, v_x, etc :tb(s=:)
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The thermo keywords allowed in the equation are those defined by the
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"thermo_style custom" command. Note that many thermodyanmic
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quantities are only computable after the first simulation has begun.
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Likewise, many thermodynamic quantities (such as energies) are only
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computed on timesteps when thermodyanmic output is being performed.
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If the variable equation these quantities at other times, out-of-date
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or invalid values may be used.
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Math functions take one or two arguments, each of which may be an
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equation containing any of the quantities defined above. This allows
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equations to be nested, as in the examples above.
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Group functions take one or two arguments. The first argument is the
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group-ID. The {dim} argument is {x} or {y} or {z}. The {dir}
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argument is {xmin}, {xmax}, {ymin}, {ymax}, {zmin}, or {zmax}. The
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group functions mass() and charge() are the total mass and charge of
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the group of atoms. Xcm() and vcm() return components of the position
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and velocity of the center of mass of the group. Fcm() returns a
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component of the total force on the group of atoms. Bound() returns
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the min/max of a particular coordinate for all atoms in the group.
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Gyration() computes the radius-of-gyration of the group of atoms. See
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the "fix gyration"_fix_gyration.html command for the formula.
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For {equal} style variables, atom vectors take a single integer
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argument from 1-N, which is the desired atom-ID, e.g. x\[243\]. For
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{atom} style variables, atom vectors take no argument. Since {atom}
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style variables compute one value per atom, a reference like x\[\]
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means the x-coord of each atom will be used when evaluating the
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variable.
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Compute references access scalar or vector quantities calculated by a
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"compute"_compute.html. 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 "compute"_compute.html command for details. Note that
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per-atom quantities calculated by a compute cannot be accessed this
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way, but only global scalar or vector quantities.
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If {c_ID\[0\]} is used as a keyword, then the scalar quantity
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calculated by the compute is printed. If {c_ID\[N\]} is used, then N
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in the range from 1-M will print the Mth component of the N-length
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vector calculated by the compute.
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The current values of other variables can be accessed by prepending a
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"v_" to the variable name. This will cause the other variable to be
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evaulated. Note that if you do something circular like this:
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variable a equal v_b
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variable b equal v_a
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print $a :pre
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then LAMMPS will run for a while when the print statement is invoked.
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Note that there is a subtle difference between using a variable in a
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{equal} or {atom} style equation in the form $x versus v_x.
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In the former case, as with any other input script command, the
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variable's value is substituted for immediately when the line is read
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from the input script. Thus if the current simulation box volume was
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1000.0, then these lines:
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variable x equal vol
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variable y equal mult($x,2) :pre
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would associate the equation string "mult(1000.0,2)" with variable y.
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By contrast, these lines:
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variable x equal vol
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variable y equal mult(v_x,2) :pre
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would associate the equation string "mult(v_x,2)" with variable y.
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Thus if the variable y were evaluated periodically during a run where
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the box volume changed, the resulting value would always be 2000.0 for
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the first case, but would change dynamically for the second case.
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:line
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[Restrictions:]
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The use of atom vectors in {equal} 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 unless
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the "atom_modify map"_atom_modify.html command is used.
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[Related commands:]
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"next"_next.html, "jump"_jump.html, "include"_include.html,
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"temper"_temper.html, "fix print"_fix_print.html, "print"_print.html
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[Default:] none
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