\(\renewcommand{\AA}{\text{Å}}\)
compute chunk/atom command
Syntax
compute ID group-ID chunk/atom style args keyword values ...
ID, group-ID are documented in compute command
chunk/atom = style name of this compute command
style = bin/1d or bin/2d or bin/3d or bin/sphere or bin/cylinder or type or molecule or c_ID, c_ID[I], f_ID, f_ID[I], v_name bin/1d args = dim origin delta dim = x or y or z origin = lower or center or upper or coordinate value (distance units) delta = thickness of spatial bins in dim (distance units) bin/2d args = dim origin delta dim origin delta dim = x or y or z origin = lower or center or upper or coordinate value (distance units) delta = thickness of spatial bins in dim (distance units) bin/3d args = dim origin delta dim origin delta dim origin delta dim = x or y or z origin = lower or center or upper or coordinate value (distance units) delta = thickness of spatial bins in dim (distance units) bin/sphere args = xorig yorig zorig rmin rmax nsbin xorig,yorig,zorig = center point of sphere srmin,srmax = bin from sphere radius rmin to rmax nsbin = # of spherical shell bins between rmin and rmax bin/cylinder args = dim origin delta c1 c2 rmin rmax ncbin dim = x or y or z = axis of cylinder axis origin = lower or center or upper or coordinate value (distance units) delta = thickness of spatial bins in dim (distance units) c1,c2 = coords of cylinder axis in other 2 dimensions (distance units) crmin,crmax = bin from cylinder radius rmin to rmax (distance units) ncbin = # of concentric circle bins between rmin and rmax type args = none molecule args = none c_ID, c_ID[I], f_ID, f_ID[I], v_name args = none c_ID = per-atom vector calculated by a compute with ID c_ID[I] = Ith column of per-atom array calculated by a compute with ID f_ID = per-atom vector calculated by a fix with ID f_ID[I] = Ith column of per-atom array calculated by a fix with ID v_name = per-atom vector calculated by an atom-style variable with namezero or more keyword/values pairs may be appended
keyword = region or nchunk or limit or ids or compress or discard or bound or pbc or units
region value = region-ID region-ID = ID of region atoms must be in to be part of a chunk nchunk value = once or every once = only compute the number of chunks once every = re-compute the number of chunks whenever invoked limit values = 0 or Nc max or Nc exact 0 = no limit on the number of chunks Nc max = limit number of chunks to be <= Nc Nc exact = set number of chunks to exactly Nc ids value = once or nfreq or every once = assign chunk IDs to atoms only once, they persist thereafter nfreq = assign chunk IDs to atoms only once every Nfreq steps (if invoked by fix ave/chunk which sets Nfreq) every = assign chunk IDs to atoms whenever invoked compress value = yes or no yes = compress chunk IDs to eliminate IDs with no atoms no = do not compress chunk IDs even if some IDs have no atoms discard value = yes or no or mixed yes = discard atoms with out-of-range chunk IDs by assigning a chunk ID = 0 no = keep atoms with out-of-range chunk IDs by assigning a valid chunk ID mixed = keep or discard such atoms according to spatial binning rule bound values = x/y/z lo hi x/y/z = x or y or z to bound spatial bins in this dimension lo = lower or coordinate value (distance units) hi = upper or coordinate value (distance units) pbc value = no or yes yes = use periodic distance for bin/sphere and bin/cylinder styles units value = box or lattice or reduced
Examples
compute 1 all chunk/atom type
compute 1 all chunk/atom bin/1d z lower 0.02 units reduced
compute 1 all chunk/atom bin/2d z lower 1.0 y 0.0 2.5
compute 1 all chunk/atom molecule region sphere nchunk once ids once compress yes
compute 1 all chunk/atom bin/sphere 5 5 5 2.0 5.0 5 discard yes
compute 1 all chunk/atom bin/cylinder z lower 2 10 10 2.0 5.0 3 discard yes
compute 1 all chunk/atom c_cluster
Description
Define a computation that calculates an integer chunk ID from 1 to Nchunk for each atom in the group. Values of chunk IDs are determined by the style of chunk, which can be based on atom type or molecule ID or spatial binning or a per-atom property or value calculated by another compute, fix, or atom-style variable. Per-atom chunk IDs can be used by other computes with “chunk” in their style name, such as compute com/chunk or compute msd/chunk. Or they can be used by the fix ave/chunk command to sum and time average a variety of per-atom properties over the atoms in each chunk. Or they can simply be accessed by any command that uses per-atom values from a compute as input, as discussed on the Howto output doc page.
See the Howto chunk page for an overview of how this compute can be used with a variety of other commands to tabulate properties of a simulation. The page gives several examples of input script commands that can be used to calculate interesting properties.
Conceptually it is important to realize that this compute does two simple things. First, it sets the value of Nchunk = the number of chunks, which can be a constant value or change over time. Second, it assigns each atom to a chunk via a chunk ID. Chunk IDs range from 1 to Nchunk inclusive; some chunks may have no atoms assigned to them. Atoms that do not belong to any chunk are assigned a value of 0. Note that the two operations are not always performed together. For example, spatial bins can be setup once (which sets Nchunk), and atoms assigned to those bins many times thereafter (setting their chunk IDs).
All other commands in LAMMPS that use chunk IDs assume there are Nchunk number of chunks, and that every atom is assigned to one of those chunks, or not assigned to any chunk.
There are many options for specifying for how and when Nchunk is calculated, and how and when chunk IDs are assigned to atoms. The details depend on the chunk style and its args, as well as optional keyword settings. They can also depend on whether a fix ave/chunk command is using this compute, since that command requires Nchunk to remain static across windows of timesteps it specifies, while it accumulates per-chunk averages.
The details are described below.
The different chunk styles operate as follows. For each style, how it calculates Nchunk and assigns chunk IDs to atoms is explained. Note that using the optional keywords can change both of those actions, as described further below where the keywords are discussed.
The binning styles perform a spatial binning of atoms, and assign an atom the chunk ID corresponding to the bin number it is in. Nchunk is set to the number of bins, which can change if the simulation box size changes. This also depends on the setting of the units keyword (e.g., for reduced units the number of chunks may not change even if the box size does).
The bin/1d, bin/2d, and bin/3d styles define bins as 1d layers (slabs), 2d pencils, or 3d boxes. The dim, origin, and delta settings are specified 1, 2, or 3 times. For 2d or 3d bins, there is no restriction on specifying dim = x before dim = y or z, or dim = y before dim = z. Bins in a particular dim have a bin size in that dimension given by delta. In each dimension, bins are defined relative to a specified origin, which may be the lower/upper edge of the simulation box (in that dimension), or its center point, or a specified coordinate value. Starting at the origin, sufficient bins are created in both directions to completely span the simulation box or the bounds specified by the optional bounds keyword.
For orthogonal simulation boxes, the bins are layers, pencils, or boxes aligned with the xyz coordinate axes. For triclinic (non-orthogonal) simulation boxes, the bin faces are parallel to the tilted faces of the simulation box. See the Howto triclinic page for a discussion of the geometry of triclinic boxes in LAMMPS. As described there, a tilted simulation box has edge vectors \(\vec a\), \(\vec b\), and \(\vec c\). In that nomenclature, bins in the x dimension have faces with normals in the \(\vec b \times \vec c\) direction, bins in y have faces normal to the \(\vec a \times \vec c\) direction, and bins in z have faces normal to the \(\vec a \times \vec b\) direction. Note that in order to define the size and position of these bins in an unambiguous fashion, the units option must be set to reduced when using a triclinic simulation box, as noted below.
The meaning of origin and delta for triclinic boxes is as follows. Consider a triclinic box with bins that are 1d layers or slabs in the x dimension. No matter how the box is tilted, an origin of 0.0 means start layers at the lower \(\vec b \times \vec c\) plane of the simulation box and an origin of 1.0 means to start layers at the upper \(\vec b \times \vec c\) face of the box. A delta value of 0.1 in reduced units means there will be 10 layers from 0.0 to 1.0, regardless of the current size or shape of the simulation box.
The bin/sphere style defines a set of spherical shell bins around the origin (xorig,yorig,zorig), using nsbin bins with radii equally spaced between srmin and srmax. This is effectively a 1d vector of bins. For example, if srmin = 1.0 and srmax = 10.0 and nsbin = 9, then the first bin spans \(1.0 < r < 2.0\), and the last bin spans \(9.0 < r < 10.0\). The geometry of the bins is the same whether the simulation box is orthogonal or triclinic (i.e., the spherical shells are not tilted or scaled differently in different dimensions to transform them into ellipsoidal shells).
The bin/cylinder style defines bins for a cylinder oriented along the axis dim with the axis coordinates in the other two radial dimensions at (c1,c2). For dim = x, \(c_1/c_2 = y/z\); for dim = y, \(c_1/c_2 = x/z\); for dim = z, \(c_1/c_2 = x/y\). This is effectively a 2d array of bins. The first dimension is along the cylinder axis, the second dimension is radially outward from the cylinder axis. The bin size and positions along the cylinder axis are specified by the origin and delta values, the same as for the bin/1d, bin/2d, and bin/3d styles. There are ncbin concentric circle bins in the radial direction from the cylinder axis with radii equally spaced between crmin and crmax. For example, if crmin = 1.0 and crmax = 10.0 and ncbin = 9, then the first bin spans \(1.0 < r < 2.0\) and the last bin spans \(9.0 < r < 10.0\). The geometry of the bins in the radial dimensions is the same whether the simulation box is orthogonal or triclinic (i.e., the concentric circles are not tilted or scaled differently in the two different dimensions to transform them into ellipses).
The created bins (and hence the chunk IDs) are numbered consecutively from 1 to the number of bins = Nchunk. For bin2d and bin3d, the numbering varies most rapidly in the first dimension (which could be x, y, or z), next rapidly in the second dimension, and most slowly in the third dimension. For bin/sphere, the bin with smallest radii is chunk 1 and the bin with largest radii is chunk Nchunk = ncbin. For bin/cylinder, the numbering varies most rapidly in the dimension along the cylinder axis and most slowly in the radial direction.
Each time this compute is invoked, each atom is mapped to a bin based on its current position. Note that between reneighboring timesteps, atoms can move outside the current simulation box. If the box is periodic (in that dimension) the atom is remapping into the periodic box for purposes of binning. If the box in not periodic, the atom may have moved outside the bounds of all bins. If an atom is not inside any bin, the discard keyword is used to determine how a chunk ID is assigned to the atom.
The type style uses the atom type as the chunk ID. Nchunk is set to the number of atom types defined for the simulation (e.g., via the create_box or read_data commands).
The molecule style uses the molecule ID of each atom as its chunk ID. Nchunk is set to the largest chunk ID. Note that this excludes molecule IDs for atoms which are not in the specified group or optional region.
There is no requirement that all atoms in a particular molecule are assigned the same chunk ID (zero or non-zero), though you probably want that to be the case, if you wish to compute a per-molecule property. LAMMPS will issue a warning if that is not the case, but only the first time that Nchunk is calculated.
Note that atoms with a molecule ID = 0, which may be non-molecular solvent atoms, have an out-of-range chunk ID. These atoms are discarded (not assigned to any chunk) or assigned to Nchunk, depending on the value of the discard keyword.
The compute/fix/variable styles set the chunk ID of each atom based on a quantity calculated and stored by a compute, fix, or variable. In each case, it must be a per-atom quantity. In each case the referenced floating point values are converted to an integer chunk ID as follows. The floating point value is truncated (rounded down) to an integer value. If the integer value is \(\le 0\), then a chunk ID of 0 is assigned to the atom. If the integer value is \(> 0\), it becomes the chunk ID to the atom. Nchunk is set to the largest chunk ID. Note that this excludes atoms which are not in the specified group or optional region.
If the style begins with “c_”, a compute ID must follow which has been previously defined in the input script. If no bracketed integer is appended, the per-atom vector calculated by the compute is used. If a bracketed integer is appended, the Ith column of the per-atom array calculated by the compute is used. Users can also write code for their own compute styles and add them to LAMMPS.
If the style begins with “f_”, a fix ID must follow which has been previously defined in the input script. If no bracketed integer is appended, the per-atom vector calculated by the fix is used. If a bracketed integer is appended, the Ith column of the per-atom array calculated by the fix is used. Note that some fixes only produce their values on certain timesteps, which must be compatible with the timestep on which this compute accesses the fix, else an error results. Users can also write code for their own fix styles and add them to LAMMPS.
If a value begins with “v_”, a variable name for an atom or atomfile style variable must follow which has been previously defined in the input script. Variables of style atom can reference thermodynamic keywords and various per-atom attributes, or invoke other computes, fixes, or variables when they are evaluated, so this is a very general means of generating per-atom quantities to treat as a chunk ID.
Normally, Nchunk = the number of chunks, is re-calculated every time this fix is invoked, though the value may or may not change. As explained below, the nchunk keyword can be set to once which means Nchunk will never change.
If a fix ave/chunk command uses this compute, it can also turn off the re-calculation of Nchunk for one or more windows of timesteps. The extent of the windows, during which Nchunk is held constant, are determined by the Nevery, Nrepeat, Nfreq values and the ave keyword setting that are used by the fix ave/chunk command.
Specifically, if ave = one, then for each span of Nfreq timesteps, Nchunk is held constant between the first timestep when averaging is done (within the Nfreq-length window), and the last timestep when averaging is done (multiple of Nfreq). If ave = running or window, then Nchunk is held constant forever, starting on the first timestep when the fix ave/chunk command invokes this compute.
Note that multiple fix ave/chunk commands can use the same compute chunk/atom compute. However, the time windows they induce for holding Nchunk constant must be identical, else an error will be generated.
The various optional keywords operate as follows. Note that some of them function differently or are ignored by different chunk styles. Some of them also have different default values, depending on the chunk style, as listed below.
The region keyword applies to all chunk styles. If used, an atom must be in both the specified group and the specified geometric region to be assigned to a chunk.
The nchunk keyword applies to all chunk styles. It specifies how often Nchunk is recalculated, which in turn can affect the chunk IDs assigned to individual atoms.
If nchunk is set to once, then Nchunk is only calculated once, the first time this compute is invoked. If nchunk is set to every, then Nchunk is re-calculated every time the compute is invoked. Note that, as described above, the use of this compute by the fix ave/chunk command can override the every setting.
The default values for nchunk are listed below and depend on the chunk style and other system and keyword settings. They attempt to represent typical use cases for the various chunk styles. The nchunk value can always be set explicitly if desired.
The limit keyword can be used to limit the calculated value of Nchunk = the number of chunks. The limit is applied each time Nchunk is calculated, which also limits the chunk IDs assigned to any atom. The limit keyword is used by all chunk styles except the binning styles, which ignore it. This is because the number of bins can be tailored using the bound keyword (described below) which effectively limits the size of Nchunk.
If limit is set to Nc = 0, then no limit is imposed on Nchunk, though the compress keyword can still be used to reduce Nchunk, as described below.
If Nc \(>\) 0, then the effect of the limit keyword depends on whether the compress keyword is also used with a setting of yes, and whether the compress keyword is specified before the limit keyword or after.
In all cases, Nchunk is first calculated in the usual way for each chunk style, as described above.
First, here is what occurs if compress yes is not set. If limit is set to Nc max, then Nchunk is reset to the smaller of Nchunk and Nc. If limit is set to Nc exact, then Nchunk is reset to Nc, whether the original Nchunk was larger or smaller than Nc. If Nchunk shrank due to the limit setting, then atom chunk IDs \(>\) Nchunk will be reset to 0 or Nchunk, depending on the setting of the discard keyword. If Nchunk grew, there will simply be some chunks with no atoms assigned to them.
If compress yes is set, and the compress keyword comes before the limit keyword, the compression operation is performed first, as described below, which resets Nchunk. The limit keyword is then applied to the new Nchunk value, exactly as described in the preceding paragraph. Note that in this case, all atoms will end up with chunk IDs \(\le\) Nc, but their original values (e.g., molecule ID or compute/fix/variable) may have been \(>\) Nc, because of the compression operation.
If compress yes is set, and the compress keyword comes after the limit keyword, then the limit value of Nc is applied first to the uncompressed value of Nchunk, but only if Nc \(<\) Nchunk (whether Nc max or Nc exact is used). This effectively means all atoms with chunk IDs \(>\) Nc have their chunk IDs reset to 0 or Nc, depending on the setting of the discard keyword. The compression operation is then performed, which may shrink Nchunk further. If the new Nchunk \(<\) Nc and limit = Nc exact is specified, then Nchunk is reset to Nc, which results in extra chunks with no atoms assigned to them. Note that in this case, all atoms will end up with chunk IDs \(\le\) Nc, and their original values (e.g., molecule ID or compute/fix/variable value) will also have been \(\le\) Nc.
The ids keyword applies to all chunk styles. If the setting is once then the chunk IDs assigned to atoms the first time this compute is invoked will be permanent, and never be re-computed.
If the setting is nfreq and if a fix ave/chunk command is using this compute, then in each of the Nchunk = constant time windows (discussed above), the chunk ID’s assigned to atoms on the first step of the time window will persist until the end of the time window.
If the setting is every, which is the default, then chunk IDs are re-calculated on any timestep this compute is invoked.
Note
If you want the persistent chunk-IDs calculated by this compute to be continuous when running from a restart file, then you should use the same ID for this compute, as in the original run. This is so that the fix this compute creates to store per-atom quantities will also have the same ID, and thus be initialized correctly with chunk IDs from the restart file.
The compress keyword applies to all chunk styles and affects how Nchunk is calculated, which in turn affects the chunk IDs assigned to each atom. It is useful for converting a “sparse” set of chunk IDs (with many IDs that have no atoms assigned to them), into a “dense” set of IDs, where every chunk has one or more atoms assigned to it.
Two possible use cases are as follows. If a large simulation box is mostly empty space, then the binning style may produce many bins with no atoms. If compress is set to yes, only bins with atoms will be contribute to Nchunk. Likewise, the molecule or compute/fix/variable styles may produce large Nchunk values. For example, the compute cluster/atom command assigns every atom an atom ID for one of the atoms it is clustered with. For a million-atom system with 5 clusters, there would only be 5 unique chunk IDs, but the largest chunk ID might be 1 million, resulting in Nchunk = 1 million. If compress is set to yes, Nchunk will be reset to 5.
If compress is set to no, which is the default, no compression is done. If it is set to yes, all chunk IDs with no atoms are removed from the list of chunk IDs, and the list is sorted. The remaining chunk IDs are renumbered from 1 to Nchunk where Nchunk is the new length of the list. The chunk IDs assigned to each atom reflect the new renumbering from 1 to Nchunk.
The original chunk IDs (before renumbering) can be accessed by the compute property/chunk command and its id keyword, or by the fix ave/chunk command which outputs the original IDs as one of the columns in its global output array. For example, using the “compute cluster/atom” command discussed above, the original 5 unique chunk IDs might be atom IDs (27,4982,58374,857838,1000000). After compression, these will be renumbered to (1,2,3,4,5). The original values (27,…,1000000) can be output to a file by the fix ave/chunk command, or by using the fix ave/time command in conjunction with the compute property/chunk command.
Note
The compression operation requires global communication across all processors to share their chunk ID values. It can require large memory on every processor to store them, even after they are compressed, if there are a large number of unique chunk IDs with atoms assigned to them. It uses a STL map to find unique chunk IDs and store them in sorted order. Each time an atom is assigned a compressed chunk ID, it must access the STL map. All of this means that compression can be expensive, both in memory and CPU time. The use of the limit keyword in conjunction with the compress keyword can affect these costs, depending on which keyword is used first. So use this option with care.
The discard keyword applies to all chunk styles. It affects what chunk IDs are assigned to atoms that do not match one of the valid chunk IDs from 1 to Nchunk. Note that it does not apply to atoms that are not in the specified group or optionally specified region. Those atoms are always assigned a chunk ID = 0.
If the calculated chunk ID for an atom is not within the range 1 to Nchunk then it is a “discard” atom. Note that Nchunk may have been shrunk by the limit keyword. Or the compress keyword may have eliminated chunk IDs that were valid before the compression took place, and are now not in the compressed list. Also note that for the molecule chunk style, if new molecules are added to the system, their chunk IDs may exceed a previously calculated Nchunk. Likewise, evaluation of a compute/fix/variable on a later timestep may return chunk IDs that are invalid for the previously calculated Nchunk.
All the chunk styles except the binning styles, must use discard set to either yes or no. If discard is set to yes, which is the default, then every “discard” atom has its chunk ID set to 0. If discard is set to no, every “discard” atom has its chunk ID set to Nchunk. I.e. it becomes part of the last chunk.
The binning styles use the discard keyword to decide whether to discard atoms outside the spatial domain covered by bins, or to assign them to the bin they are nearest to.
For the bin/1d, bin/2d, bin/3d styles the details are as follows. If discard is set to yes, an out-of-domain atom will have its chunk ID set to 0. If discard is set to no, the atom will have its chunk ID set to the first or last bin in that dimension. If discard is set to mixed, which is the default, it will only have its chunk ID set to the first or last bin if bins extend to the simulation box boundary in that dimension. This is the case if the bound keyword settings are lower and upper, which is the default. If the bound keyword settings are numeric values, then the atom will have its chunk ID set to 0 if it is outside the bounds of any bin. Note that in this case, it is possible that the first or last bin extends beyond the numeric bounds settings, depending on the specified origin. If this is the case, the chunk ID of the atom is only set to 0 if it is outside the first or last bin, not if it is simply outside the numeric bounds setting.
For the bin/sphere style the details are as follows. If discard is set to yes, an out-of-domain atom will have its chunk ID set to 0. If discard is set to no or mixed, the atom will have its chunk ID set to the first or last bin, i.e. the innermost or outermost spherical shell. If the distance of the atom from the origin is less than rmin, it will be assigned to the first bin. If the distance of the atom from the origin is greater than rmax, it will be assigned to the last bin.
For the bin/cylinder style the details are as follows. If discard is set to yes, an out-of-domain atom will have its chunk ID set to 0. If discard is set to no, the atom will have its chunk ID set to the first or last bin in both the radial and axis dimensions. If discard is set to mixed, which is the default, the radial dimension is treated the same as for discard = no. But for the axis dimension, it will only have its chunk ID set to the first or last bin if bins extend to the simulation box boundary in the axis dimension. This is the case if the bound keyword settings are lower and upper, which is the default. If the bound keyword settings are numeric values, then the atom will have its chunk ID set to 0 if it is outside the bounds of any bin. Note that in this case, it is possible that the first or last bin extends beyond the numeric bounds settings, depending on the specified origin. If this is the case, the chunk ID of the atom is only set to 0 if it is outside the first or last bin, not if it is simply outside the numeric bounds setting.
If discard is set to no or mixed, the atom will have its chunk ID set to the first or last bin, i.e. the innermost or outermost spherical shell. If the distance of the atom from the origin is less than rmin, it will be assigned to the first bin. If the distance of the atom from the origin is greater than rmax, it will be assigned to the last bin.
The bound keyword only applies to the bin/1d, bin/2d, bin/3d styles and to the axis dimension of the bin/cylinder style; otherwise it is ignored. It can be used one or more times to limit the extent of bin coverage in a specified dimension, i.e. to only bin a portion of the box. If the lo setting is lower or the hi setting is upper, the bin extent in that direction extends to the box boundary. If a numeric value is used for lo and/or hi, then the bin extent in the lo or hi direction extends only to that value, which is assumed to be inside (or at least near) the simulation box boundaries, though LAMMPS does not check for this. Note that using the bound keyword typically reduces the total number of bins and thus the number of chunks Nchunk.
The pbc keyword only applies to the bin/sphere and bin/cylinder styles. If set to yes, the distance an atom is from the sphere origin or cylinder axis is calculated in a minimum image sense with respect to periodic dimensions, when determining which bin the atom is in. I.e. if x is a periodic dimension and the distance between the atom and the sphere center in the x dimension is greater than 0.5 * simulation box length in x, then a box length is subtracted to give a distance < 0.5 * simulation box length. This allosws the sphere or cylinder center to be near a box edge, and atoms on the other side of the periodic box will still be close to the center point/axis. Note that with a setting of yes, the outer sphere or cylinder radius must also be <= 0.5 * simulation box length in any periodic dimension except for the cylinder axis dimension, or an error is generated.
The units keyword only applies to the binning styles; otherwise it is ignored. For the bin/1d, bin/2d, bin/3d styles, it determines the meaning of the distance units used for the bin sizes delta and for origin and bounds values if they are coordinate values. For the bin/sphere style it determines the meaning of the distance units used for xorig,yorig,zorig and the radii srmin and srmax. For the bin/cylinder style it determines the meaning of the distance units used for delta,c1,c2 and the radii crmin and crmax.
For orthogonal simulation boxes, any of the 3 options may be used. For non-orthogonal (triclinic) simulation boxes, only the reduced option may be used.
A box value selects standard distance units as defined by the units command (e.g., \(\AA\) for units = real or metal). A lattice value means the distance units are in lattice spacings. The lattice command must have been previously used to define the lattice spacing. A reduced value means normalized unitless values between 0 and 1, which represent the lower and upper faces of the simulation box respectively. Thus an origin value of 0.5 means the center of the box in any dimension. A delta value of 0.1 means 10 bins span the box in that dimension.
Note that for the bin/sphere style, the radii srmin and srmax are scaled by the lattice spacing or reduced value of the x dimension.
Note that for the bin/cylinder style, the radii crmin and crmax are scaled by the lattice spacing or reduced value of the first dimension perpendicular to the cylinder axis (e.g., y for an x-axis cylinder, x for a y-axis cylinder, and x for a z-axis cylinder).
Output info
This compute calculates a per-atom vector (the chunk ID), which can be accessed by any command that uses per-atom values from a compute as input. It also calculates a global scalar (the number of chunks), which can be similarly accessed everywhere outside of a per-atom context. See the Howto output page for an overview of LAMMPS output options.
The per-atom vector values are unitless chunk IDs, ranging from 1 to Nchunk (inclusive) for atoms assigned to chunks, and 0 for atoms not belonging to a chunk. The scalar contains the value of Nchunk.
Restrictions
Even if the nchunk keyword is set to once, the chunk IDs assigned to each atom are not stored in a restart files. This means you cannot expect those assignments to persist in a restarted simulation. Instead you must re-specify this command and assign atoms to chunks when the restarted simulation begins.
Default
The option defaults are as follows:
region = none
nchunk = every, if compress is yes, overriding other defaults listed here
nchunk = once, for type style
nchunk = once, for mol style if region is none
nchunk = every, for mol style if region is set
nchunk = once, for binning style if the simulation box size is static or units = reduced
nchunk = every, for binning style if the simulation box size is dynamic and units is lattice or box
nchunk = every, for compute/fix/variable style
limit = 0
ids = every
compress = no
discard = yes, for all styles except binning
discard = mixed, for binning styles
bound = lower and upper in all dimensions
pbc = no
units = lattice