forked from lijiext/lammps
140 lines
6.2 KiB
Plaintext
140 lines
6.2 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 = calculate property every this many timesteps :l
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Nrepeat = # of times to repeat the Nevery calculation before averaging :l
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Nfreq = timestep frequency at which the average value is calculated :l
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one or more values can be listed :l
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value = x, y, z, xu, yu, zu, vx, vy, vz, fx, fy, fz, c_ID, c_ID\[N\], f_ID, f_ID\[N\], v_name :l
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x,y,z,xu,yu,zu,vx,vy,vz,fx,fy,fz = atom attribute (position, unwrapped position, velocity, force component)
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c_ID = per-atom scalar value calculated by a compute with ID
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c_ID\[N\] = Nth component of per-atom vector calculated by a compute with ID
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f_ID = per-atom scalar value calculated by a fix with ID
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f_ID\[N\] = Nth component of per-atom vector calculated by a fix with ID
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v_name = per-atom value 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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Calculate one or more instantaneous per-atom quantities every few
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timesteps, and average them 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#4_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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Each listed value is averaged independently. The group specified with
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the command means only atoms within the group have their averages
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computed. Atoms not in the group have their result(s) set to 0.0.
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Each listed value can be an atom attribute (position, unwrapped
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position, velocity, force component) or can be the result of a
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"compute"_compute.html or "fix"_fix.html or the evaluation of an
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atom-style "variable"_variable.html. In the latter cases, the
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compute, fix, or variable must produce a per-atom quantity, not a
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global 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 quantities are those
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which have the word {atom} in their style name. See the doc pages for
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individual "fixes"_fix.html to determine which ones produce per-atom
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quantities. "Variables"_variable.html of style {atom} are the only
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ones that can be used with this fix since all other styles of variable
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produce global quantities.
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:line
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The {Nevery}, {Nrepeat}, and {Nfreq} arguments specify on what
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timesteps the values will be generated in order to contribute to the
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average. The final averaged quantities are generated every {Nfreq}
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timesteps. The average is over {Nrepeat} quantities, computed in the
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preceding portion of the simulation every {Nevery} timesteps. {Nfreq}
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must be a multiple of {Nevery} and {Nevery} must be non-zero even if
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{Nrepeat} is 1. Also, the timesteps contributing to the average value
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cannot overlap, i.e. Nfreq > (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. The unwrapped values (xu,yu,zu) mean that the
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coordinates are "unwrapped" by the images flags for each atom. This
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means that if the atom has passed thru a periodic boundary one or more
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times, its unwrapped value is what the coordinate would be if it had
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not been wrapped back into the periodic box. Note that this means the
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coordinate value may be far outside the box. The
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"dump_modify"_dump_modify.html command describes in more detail what
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is meant by image flags.
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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 scalar calculated by the compute is used. If a
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bracketed term is appended, the Nth vector per-atom value calculated
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by the compute is used. Users can also write code for their own
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compute styles and "add them to 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 scalar calculated by the fix is used. If a
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bracketed term is appended, the Nth vector per-atom value calculated
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by the fix is used. Note that some fixes only produce their values on
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certain timesteps, which must be compatible with {Nevery}, else an
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error will results. Users can also write code for their own fix
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styles and "add them to 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. Variables of style
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{atom} can reference thermodynamic keywords, or invoke other computes,
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fixes, or variables when they are evaluated, so this is a very general
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means of generating per-atom quantities to time average.
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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#4_15.
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This fix produces a per-atom scalar or vector which can be accessed by
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various "output commands"_Section_howto.html#4_15. A scalar 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 a vector 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/spatial"_fix_ave_spatial.html, "dump
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custom"_dump.html
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[Default:] none
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