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# read_dump command¶

## Syntax¶

read_dump file Nstep field1 field2 ... keyword values ...
• file = name of dump file to read

• Nstep = snapshot timestep to read from file

• one or more fields may be appended

field = x or y or z or vx or vy or vz or q or ix or iy or iz or fx or fy or fz
x,y,z = atom coordinates
vx,vy,vz = velocity components
q = charge
ix,iy,iz = image flags in each dimension
fx,fy,fz = force components
• zero or more keyword/value pairs may be appended

• keyword = nfile or box or timestep or replace or purge or trim or add or label or scaled or wrapped or format

nfile value = Nfiles = how many parallel dump files exist
box value = yes or no = replace simulation box with dump box
timestep value = yes or no = reset simulation timestep with dump timestep
replace value = yes or no = overwrite atoms with dump atoms
purge value = yes or no = delete all atoms before adding dump atoms
trim value = yes or no = trim atoms not in dump snapshot
add value = yes or keep or no = add new dump atoms to system
label value = field column
field = one of the listed fields or id or type
column = label on corresponding column in dump file
scaled value = yes or no = coords in dump file are scaled/unscaled
wrapped value = yes or no = coords in dump file are wrapped/unwrapped
format values = format of dump file, must be last keyword if used
native = native LAMMPS dump file
xyz = XYZ file
adios [timeout value] = dump file written by the dump adios command
timeout = specify waiting time for the arrival of the timestep when running concurrently.
The value is a float number and is interpreted in seconds.
molfile style path = VMD molfile plugin interface
style = dcd or xyz or others supported by molfile plugins
path = optional path for location of molfile plugins

## Examples¶

read_dump dump.file 5000 x y z
read_dump dump.xyz 5 x y z box no format xyz
read_dump dump.xyz 10 x y z box no format molfile xyz "../plugins"
read_dump dump.dcd 0 x y z box yes format molfile dcd
read_dump dump.file 1000 x y z vx vy vz box yes format molfile lammpstrj /usr/local/lib/vmd/plugins/LINUXAMD64/plugins/molfile
read_dump dump.file 5000 x y vx vy trim yes
read_dump dump.file 5000 x y vx vy add yes box no timestep no
read_dump ../run7/dump.file.gz 10000 x y z box yes
read_dump dump.xyz 10 x y z box no format molfile xyz ../plugins
read_dump dump.dcd 0 x y z format molfile dcd
read_dump dump.file 1000 x y z vx vy vz format molfile lammpstrj /usr/local/lib/vmd/plugins/LINUXAMD64/plugins/molfile
read_dump dump.bp 5000 x y z vx vy vz format adios
read_dump dump.bp 5000 x y z vx vy vz format adios timeout 60.0

## Description¶

Read atom information from a dump file to overwrite the current atom coordinates, and optionally the atom velocities and image flags, the simulation timestep, and the simulation box dimensions. This is useful for restarting a run from a particular snapshot in a dump file. See the read_restart and read_data commands for alternative methods to do this. Also see the rerun command for a means of reading multiple snapshots from a dump file.

Note that a simulation box must already be defined before using the read_dump command. This can be done by the create_box, read_data, or read_restart commands. The read_dump command can reset the simulation box dimensions, as explained below.

Also note that reading per-atom information from a dump snapshot is limited to the atom coordinates, velocities and image flags, as explained below. Other atom properties, which may be necessary to run a valid simulation, such as atom charge, or bond topology information for a molecular system, are not read from (or may not even be contained in) dump files. Thus this auxiliary information should be defined in the usual way, e.g. in a data file read in by a read_data command, before using the read_dump command, or by the set command, after the dump snapshot is read.

If the dump filename specified as file ends with “.gz”, the dump file is read in gzipped format.

You can read dump files that were written (in parallel) to multiple files via the “%” wild-card character in the dump file name. If any specified dump file name contains a “%”, they must all contain it. See the dump command for details. The “%” wild-card character is only supported by the native format for dump files, described next.

If reading parallel dump files, you must also use the nfile keyword to tell LAMMPS how many parallel files exist, via its specified Nfiles value.

The format of the dump file is selected through the format keyword. If specified, it must be the last keyword used, since all remaining arguments are passed on to the dump reader. The native format is for native LAMMPS dump files, written with a dump atom or dump custom command. The xyz format is for generic XYZ formatted dump files. These formats take no additional values.

The molfile format supports reading data through using the VMD molfile plugin interface. This dump reader format is only available, if the MOLFILE package has been installed when compiling LAMMPS.

The molfile format takes one or two additional values. The style value determines the file format to be used and can be any format that the molfile plugins support, such as DCD or XYZ. Note that DCD dump files can be written by LAMMPS via the dump dcd command. The path value specifies a list of directories which LAMMPS will search for the molfile plugins appropriate to the specified style. The syntax of the path value is like other search paths: it can contain multiple directories separated by a colon (or semi-colon on windows). The path keyword is optional and defaults to “.”, i.e. the current directory.

The adios format supports reading data that was written by the dump adios command. The entire dump is read in parallel across all the processes, dividing the atoms evenly among the processes. The number of writers that has written the dump file does not matter. Using the adios style for dump and read_dump is a convenient way to dump all atoms from N writers and read it back by M readers. If one is running two LAMMPS instances concurrently where one dumps data and the other is reading it with the rerun command, the timeout option can be specified to wait on the reader side for the arrival of the requested step.

Support for other dump format readers may be added in the future.

Global information is first read from the dump file, namely timestep and box information.

The dump file is scanned for a snapshot with a timestamp that matches the specified Nstep. This means the LAMMPS timestep the dump file snapshot was written on for the native or adios formats.

The list of timestamps available in an adios .bp file is stored in the variable ntimestep:

$bpls dump.bp -d ntimestep uint64_t ntimestep 5*scalar (0) 0 50 100 150 200 Note that the xyz and molfile formats do not store the timestep. For these formats, timesteps are numbered logically, in a sequential manner, starting from 0. Thus to access the 10th snapshot in an xyz or mofile formatted dump file, use Nstep = 9. The dimensions of the simulation box for the selected snapshot are also read; see the box keyword discussion below. For the native format, an error is generated if the snapshot is for a triclinic box and the current simulation box is orthogonal or vice versa. A warning will be generated if the snapshot box boundary conditions (periodic, shrink-wrapped, etc) do not match the current simulation boundary conditions, but the boundary condition information in the snapshot is otherwise ignored. See the “boundary” command for more details. The adios reader does the same as the native format reader. For the xyz format, no information about the box is available, so you must set the box flag to no. See details below. For the molfile format, reading simulation box information is typically supported, but the location of the simulation box origin is lost and no explicit information about periodicity or orthogonal/triclinic box shape is available. The MOLFILE package makes a best effort to guess based on heuristics, but this may not always work perfectly. Per-atom information from the dump file snapshot is then read from the dump file snapshot. This corresponds to the specified fields listed in the read_dump command. It is an error to specify a z-dimension field, namely z, vz, or iz, for a 2d simulation. For dump files in native format, each column of per-atom data has a text label listed in the file. A matching label for each field must appear, e.g. the label “vy” for the field vy. For the x, y, z fields any of the following labels are considered a match: x, xs, xu, xsu for field x y, ys, yu, ysu for field y z, zs, zu, zsu for field z The meaning of xs (scaled), xu (unwrapped), and xsu (scaled and unwrapped) is explained on the dump command doc page. These labels are searched for in the list of column labels in the dump file, in order, until a match is found. The dump file must also contain atom IDs, with a column label of “id”. If the add keyword is specified with a value of yes or keep, as discussed below, the dump file must contain atom types, with a column label of “type”. If a column label you want to read from the dump file is not a match to a specified field, the label keyword can be used to specify the specific column label from the dump file to associate with that field. An example is if a time-averaged coordinate is written to the dump file via the fix ave/atom command. The column will then have a label corresponding to the fix-ID rather than “x” or “xs”. The label keyword can also be used to specify new column labels for fields id and type. For dump files in xyz format, only the x, y, and z fields are supported. The dump file does not store atom IDs, so these are assigned consecutively to the atoms as they appear in the dump file, starting from 1. Thus you should insure that order of atoms is consistent from snapshot to snapshot in the XYZ dump file. See the dump_modify sort command if the XYZ dump file was written by LAMMPS. For dump files in molfile format, the x, y, z, vx, vy, and vz fields can be specified. However, not all molfile formats store velocities, or their respective plugins may not support reading of velocities. The molfile dump files do not store atom IDs, so these are assigned consecutively to the atoms as they appear in the dump file, starting from 1. Thus you should insure that order of atoms are consistent from snapshot to snapshot in the molfile dump file. See the dump_modify sort command if the dump file was written by LAMMPS. The adios format supports all fields that the native format supports except for the q charge field. The list of fields stored in an adios .bp file is recorded in the attributes columns (array of short strings) and columnstr (space-separated single string).$ bpls -la dump.bp column*
string    columns            attr   = {"id", "type", "x", "y", "z", "vx", "vy", "vz"}
string    columnstr          attr   = "id type x y z vx vy vz "

Information from the dump file snapshot is used to overwrite or replace properties of the current system. There are various options for how this is done, determined by the specified fields and optional keywords.

Changed in version 3Aug2022.

The timestep of the snapshot becomes the current timestep for the simulation unless the timestep keyword is specified with a no value (default setting is yes). See the reset_timestep command if you wish to change this to a different value after the dump snapshot is read.

If the box keyword is specified with a yes value, then the current simulation box dimensions are replaced by the dump snapshot box dimensions. If the box keyword is specified with a no value, the current simulation box is unchanged.

If the purge keyword is specified with a yes value, then all current atoms in the system are deleted before any of the operations invoked by the replace, trim, or add keywords take place.

If the replace keyword is specified with a yes value, then atoms with IDs that are in both the current system and the dump snapshot have their properties overwritten by field values. If the replace keyword is specified with a no value, atoms with IDs that are in both the current system and the dump snapshot are not modified.

If the trim keyword is specified with a yes value, then atoms with IDs that are in the current system but not in the dump snapshot are deleted. These atoms are unaffected if the trim keyword is specified with a no value.

If the add keyword is specified with a no value (default), then dump file atoms with IDs that are not in the current system are not added to the system. They are simply ignored.

If a yes value is specified, the atoms with new IDs are added to the system but their atom IDs are not preserved. Instead, after all the atoms are added, new IDs are assigned to them in the same manner as is described for the create_atoms command. Basically the largest existing atom ID in the system is identified, and all the added atoms are assigned IDs that consecutively follow the largest ID.

If a keep value is specified, the atoms with new IDs are added to the system and their atom IDs are preserved. This may lead to non-contiguous IDs for the combined system.

Note that atoms added via the add keyword will only have the attributes read from the dump file due to the field arguments. For example, if x or y or z or q is not specified as a field, a value of 0.0 is used for added atoms. Added atoms must have an atom type, so this value must appear in the dump file.

Any other attributes (e.g. charge or particle diameter for spherical particles) will be set to default values, the same as if the create_atoms command were used.

Atom coordinates read from the dump file are first converted into unscaled coordinates, relative to the box dimensions of the snapshot. These coordinates are then be assigned to an existing or new atom in the current simulation. The coordinates will then be remapped to the simulation box, whether it is the original box or the dump snapshot box. If periodic boundary conditions apply, this means the atom will be remapped back into the simulation box if necessary. If shrink-wrap boundary conditions apply, the new coordinates may change the simulation box dimensions. If fixed boundary conditions apply, the atom will be lost if it is outside the simulation box.

For native format dump files, the 3 xyz image flags for an atom in the dump file are set to the corresponding values appearing in the dump file if the ix, iy, iz fields are specified. If not specified, the image flags for replaced atoms are not changed and image flags for new atoms are set to default values. If coordinates read from the dump file are in unwrapped format (e.g. xu) then the image flags for read-in atoms are also set to default values. The remapping procedure described in the previous paragraph will then change images flags for all atoms (old and new) if periodic boundary conditions are applied to remap an atom back into the simulation box.

Note

If you get a warning about inconsistent image flags after reading in a dump snapshot, it means one or more pairs of bonded atoms now have inconsistent image flags. As discussed on the Errors common page this may or may not cause problems for subsequent simulations. One way this can happen is if you read image flag fields from the dump file but do not also use the dump file box parameters.

LAMMPS knows how to compute unscaled and remapped coordinates for the snapshot column labels discussed above, e.g. x, xs, xu, xsu. If another column label is assigned to the x or y or z field via the label keyword, e.g. for coordinates output by the fix ave/atom command, then LAMMPS needs to know whether the coordinate information in the dump file is scaled and/or wrapped. This can be set via the scaled and wrapped keywords. Note that the value of the scaled and wrapped keywords is ignored for fields x or y or z if the label keyword is not used to assign a column label to that field.

The scaled/unscaled and wrapped/unwrapped setting must be identical for any of the x, y, z fields that are specified. Thus you cannot read xs and yu from the dump file. Also, if the dump file coordinates are scaled and the simulation box is triclinic, then all 3 of the x, y, z fields must be specified, since they are all needed to generate absolute, unscaled coordinates.

## Restrictions¶

To read gzipped dump files, you must compile LAMMPS with the -DLAMMPS_GZIP option. See the Build settings doc page for details.

The molfile dump file formats are part of the MOLFILE package. They are only enabled if LAMMPS was built with that packages. See the Build package page for more info.

To write and read adios .bp files, you must compile LAMMPS with the ADIOS package.

## Default¶

The option defaults are box = yes, timestep = yes, replace = yes, purge = no, trim = no, add = no, scaled = no, wrapped = yes, and format = native.