forked from lijiext/lammps
154 lines
6.5 KiB
Plaintext
154 lines
6.5 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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fix ave/atom command :h3
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[Syntax:]
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fix ID group-ID ave/atom Nevery Nrepeat Nfreq value1 value2 ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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ave/atom = style name of this fix command :l
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Nevery = use input values every this many timesteps :l
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Nrepeat = # of times to use input values for calculating averages :l
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Nfreq = calculate averages every this many timesteps
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one or more input values can be listed :l
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value = x, y, z, vx, vy, vz, fx, fy, fz, c_ID, c_ID\[i\], f_ID, f_ID\[i\], v_name :l
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x,y,z,vx,vy,vz,fx,fy,fz = atom attribute (position, velocity, force component)
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c_ID = per-atom vector calculated by a compute with ID
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c_ID\[I\] = Ith column of per-atom array calculated by a compute with ID
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f_ID = per-atom vector calculated by a fix with ID
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f_ID\[I\] = Ith column of per-atom array calculated by a fix with ID
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v_name = per-atom vector calculated by an atom-style variable with name :pre
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:ule
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[Examples:]
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fix 1 all ave/atom 1 100 100 vx vy vz
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fix 1 all ave/atom 10 20 1000 c_my_stress\[1\] :pre
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[Description:]
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Use one or more per-atom vectors as inputs every few timesteps, and
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average them atom by atom over longer timescales. The resulting
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per-atom averages can be used by other "output
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commands"_Section_howto.html#howto_15 such as the "fix
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ave/spatial"_fix_ave_spatial.html or "dump custom"_dump.html commands.
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The group specified with the command means only atoms within the group
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have their averages computed. Results are set to 0.0 for atoms not in
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the group.
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Each input value can be an atom attribute (position, velocity, force
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component) or can be the result of a "compute"_compute.html or
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"fix"_fix.html or the evaluation of an atom-style
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"variable"_variable.html. In the latter cases, the compute, fix, or
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variable must produce a per-atom vector, not a global quantity or
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local quantity. If you wish to time-average global quantities from a
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compute, fix, or variable, then see the "fix
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ave/time"_fix_ave_time.html command.
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"Computes"_compute.html that produce per-atom vectors or arrays are
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those which have the word {atom} in their style name. See the doc
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pages for individual "fixes"_fix.html to determine which ones produce
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per-atom vectors or arrays. "Variables"_variable.html of style {atom}
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are the only ones that can be used with this fix since they produce
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per-atom vectors.
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Each per-atom value of each input vector is averaged independently.
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:line
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The {Nevery}, {Nrepeat}, and {Nfreq} arguments specify on what
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timesteps the input values will be used in order to contribute to the
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average. The final averaged quantities are generated on timesteps
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that are a multiple of {Nfreq}. The average is over {Nrepeat}
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quantities, computed in the preceding portion of the simulation every
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{Nevery} timesteps. {Nfreq} must be a multiple of {Nevery} and
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{Nevery} must be non-zero even if {Nrepeat} is 1. Also, the timesteps
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contributing to the average value cannot overlap, i.e. Nfreq >
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(Nrepeat-1)*Nevery is required.
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For example, if Nevery=2, Nrepeat=6, and Nfreq=100, then values on
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timesteps 90,92,94,96,98,100 will be used to compute the final average
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on timestep 100. Similarly for timesteps 190,192,194,196,198,200 on
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timestep 200, etc.
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:line
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The atom attribute values (x,y,z,vx,vy,vz,fx,fy,fz) are
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self-explanatory. Note that other atom attributes can be used as
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inputs to this fix by using the "compute
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property/atom"_compute_property_atom.html command and then specifying
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an input value from that compute.
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IMPORTANT NOTE: The x,y,z attributes are values that are re-wrapped
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inside the periodic box whenever an atom crosses a periodic boundary.
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Thus if you time average an atom that spends half its time on either
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side of the periodic box, you will get a value in the middle of the
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box. If this is not what you want, consider averaging unwrapped
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coordinates, which can be provided by the "compute
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property/atom"_compute_property_atom.html command via its xu,yu,zu
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attributes.
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If a value begins with "c_", a compute ID must follow which has been
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previously defined in the input script. If no bracketed term is
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appended, the per-atom vector calculated by the compute is used. If a
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bracketed term containing an index I is appended, the Ith column of
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the per-atom array calculated by the compute is used. Users can also
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write code for their own compute styles and "add them to
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LAMMPS"_Section_modify.html.
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If a value begins with "f_", a fix ID must follow which has been
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previously defined in the input script. If no bracketed term is
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appended, the per-atom vector calculated by the fix is used. If a
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bracketed term containing an index I is appended, the Ith column of
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the per-atom array calculated by the fix is used. Note that some
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fixes only produce their values on certain timesteps, which must be
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compatible with {Nevery}, else an error will result. Users can also
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write code for their own fix styles and "add them to
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LAMMPS"_Section_modify.html.
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If a value begins with "v_", a variable name must follow which has
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been previously defined in the input script as an "atom-style
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variable"_variable.html Variables of style {atom} can reference
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thermodynamic keywords, or invoke other computes, fixes, or variables
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when they are evaluated, so this is a very general means of generating
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per-atom quantities to time average.
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:line
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about this fix is written to "binary restart
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files"_restart.html. None of the "fix_modify"_fix_modify.html options
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are relevant to this fix. No global scalar or vector quantities are
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stored by this fix for access by various "output
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commands"_Section_howto.html#howto_15.
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This fix produces a per-atom vector or array which can be accessed by
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various "output commands"_Section_howto.html#howto_15. A vector is
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produced if only a single quantity is averaged by this fix. If two or
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more quantities are averaged, then an array of values is produced.
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The per-atom values can only be accessed on timesteps that are
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multiples of {Nfreq} since that is when averaging is performed.
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No parameter of this fix can be used with the {start/stop} keywords of
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the "run"_run.html command. This fix is not invoked during "energy
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minimization"_minimize.html.
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[Restrictions:] none
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[Related commands:]
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"compute"_compute.html, "fix ave/histo"_fix_ave_histo.html, "fix
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ave/spatial"_fix_ave_spatial.html, "fix ave/time"_fix_ave_time.html,
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"variable"_variable.html,
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[Default:] none
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