forked from lijiext/lammps
143 lines
6.4 KiB
Plaintext
143 lines
6.4 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/spatial command :h3
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[Syntax:]
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fix ID group-ID ave/spatial Nevery Nfreq dim origin delta file style args keyword value ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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ave/spatial = style name of this fix command :l
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Nevery = calculate property every this many timesteps :l
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Nfreq = write average property to file every this many steps :l
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dim = {x} or {y} or {z} :l
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origin = {lower} or {center} or {upper} or coordinate value (distance units) :l
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delta = thickness of spatial layers in dim (distance units) :l
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file = filename to write results to :l
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style = {density} or {atom} or {compute} :l
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{density} arg = {mass} or {number}
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{mass} = compute mass density
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{number} = compute number density
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{atom} arg = {vx} or {vy} or {vz} or {fx} or {fy} or {fz}
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{compute} arg = compute-ID that calculates per-atom quantities :pre
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zero or more keyword/value pairs may be appended to the args :l
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keyword = {norm} or {units}
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{norm} value = {all} or {sample}
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{units} value = {box} or {lattice} :pre
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:ule
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[Examples:]
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fix 1 all ave/spatial 10000 10000 z lower 2.0 centro.profile compute myCentro
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fix 1 flow ave/spatial 100 1000 y 0.0 1.0 vel.profile atom vx norm sample
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fix 1 flow ave/spatial 100 1000 y 0.0 1.0 dens.profile density mass :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, average them by layer in a chosen dimension and over a
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longer timescale, and print the results to a file. This can be used
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to spatially average per-atom properties such as velocity or energy or
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a quantity calculated by an equation you define; see the "variable
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atom"_variable.html command.
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The {density} styles means to simply count the number of atoms in each
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layer, either by mass or number. The {atom} style allows an atom
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property such as x-velocity to be specified. The {compute} style
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allows specification of a "compute"_compute.html which will be invoked
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to calculate the desired property. The compute can be previously
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defined in the input script. Note that the "compute variable/atom"
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style allows you to calculate any quantity for an atom that can be
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specified by a "variable atom"_variable.html equation. 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. Note that the "dump custom"_dump.html
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command can also be used to output per-atom quantities calculated by a
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compute.
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For the {compute} style, the fix ave/spatial style uses the per-atom
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scalar or vector calculated by the compute. See the "fix
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ave/time"_fix_ave_time.html command if you wish to time-average a
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global quantity, e.g. via a compute that temperature or pressure.
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In all cases, the calculated property is averaged over atoms in each
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layer, where the layers are in a particular {dim} and have a thickness
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given by {delta}. Every Nfreq steps, when a property is calculated
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for the first time (after a previous write), the number of layers and
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the layer boundaries are computed. Thus if the simlation box changes
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size during a simulation, the number of layers and their boundaries
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may also change. Layers are defined relative to a specified {origin},
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which may be the lower/upper edge of the box (in {dim}) or its center
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point, or a specified coordinate value. Starting at the origin,
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sufficient layers are created in both directions to completely cover
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the box. On subsequent timesteps every atom is mapped to one of the
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layers. Atoms beyond the lowermost/uppermost layer are counted in the
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first/last layer.
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The {units} keyword determines the meaning of the distance units used
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for the layer thickness {delta} and {origin} if it is a coordinate
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value. A {box} value selects standard distance units as defined by
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the "units"_units.html command, e.g. Angstroms for units = real or
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metal. A {lattice} value means the distance units are in lattice
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spacings. The "lattice"_lattice.html command must have been
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previously used to define the lattice spacing.
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The {Nevery} and {Nfreq} arguments specify how the property calculated
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for each layer is time-averaged. The property is calculated once each
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Nevery timesteps. It is averaged and output every Nfreq timesteps.
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Nfreq must be a multiple of Nevery. In the 2nd example above, the
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property is calculated every 100 steps. After 10 calculations, the
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average result is written to the file, once every 1000 steps.
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The {norm} keyword also affects how time-averaging is done. For an
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{all} setting, a layer quantity is summed over all atoms in all
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Nfreq/Nevery samples, as is the count of atoms in the layer. The
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printed value for the layer is Total-quantity / Total-count.
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In other words it is an average over the entire Nfreq timescale.
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For a {sample} setting, the quantity is summed over atoms for only a
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single sample, as is the count, and a "average sample value" is
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computed, i.e. Sample-quantity / Sample-count. The printed value for
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the layer is the average of the M "average sample values", where M =
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Nfreq/Nevery. In other words it is an average of an average.
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Each time info is written to the file, it is in the following format.
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A line with the timestep and number of layers is written. Then one
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line per layer is written, containing the layer ID (1-N), the
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coordinate of the center of the layer, the number of atoms in the
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layer, and one or more calculated values. The number of atoms and the
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value(s) are average quantities.
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If the {density} or {atom} keyword is used, or the {compute} keyword
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with a compute that calculates a single quantity per atom, then a
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single value will be printed for each layer. If the {compute} keyword
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is used with a compute that calculates N quantities per atom, then N
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values per line will be written, each of them averaged independently.
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For the {compute} keyword, the calculation performed by the compute in
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on the group defined by the compute. However, only atoms in the fix
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group are included in the layer averaging. LAMMPS prints a warning if
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the fix group and compute group do not match.
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Note that some computes perform costly calculations, involving use of
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or creation of neighbor lists. If the compute is invoked too often by
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fix ave/spatial, it can slow down a simulation.
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[Restrictions:] none
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[Related commands:]
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"compute"_compute.html, "fix ave/time"_fix_ave_time.html
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[Default:]
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The option defaults are norm = all and units = lattice.
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