lammpsio 0.6.1

Python tools for working with LAMMPS

lammpsio

Tools for working with LAMMPS data and dump files.

lammpsio is a pure Python package that can be installed using pip:

pip install lammpsio

or conda:

conda install -c conda-forge lammpsio

Snapshot

The particle configuration is stored in a Snapshot. A Snapshot holds the data for N particles, the simulation Box, and the timestep. The Box follows the LAMMPS conventions for its shape and bounds. Here is a 3-particle configuration in an orthorhombic box centered at the origin at step 100:

box = lammpsio.Box((-2,-3,-4), (2,3,4))
snapshot = lammpsio.Snapshot(3, box, step=100)

These constructor arguments are available as attributes:

• N: number of particles (int)
• box: bounding box (Box)
• step: timestep counter (int)
• num_types: number of particle types (int). If num_types is None, then the number of types is deduced from typeid.

The data contained in a Snapshot per particle is:

• id: (N,) array atom IDs (dtype: int, default: runs from 1 to N)
• position: (N,3) array of coordinates (dtype: float, default: (0,0,0))
• image: (N,3) array of periodic image indexes (dtype: int, default: (0,0,0))
• velocity: (N,3) array of velocities (dtype: float, default: (0,0,0))
• molecule: (N,) array of molecule indexes (dtype: int, default: 0)
• typeid: (N,) array of type indexes (dtype: int, default: 1)
• mass: (N,) array of masses (dtype: float, default: 1)
• charge: (N,) array of charges (dtype: float, default: 0)
• bonds: Bond data (dtype: Bonds, default: None)
• angles: Angle data (dtype: Angles, default: None)
• dihedrals: Dihedral data (dtype: Dihedrals, default: None)
• impropers: Improper data (dtype: Impropers, default: None)

All values of indexes will follow the LAMMPS 1-indexed convention, but the arrays themselves are 0-indexed.

The Snapshot will lazily initialize these per-particle arrays as they are accessed to save memory. Hence, accessing a per-particle property will allocate it to default values. If you want to check if an attribute has been set, use the corresponding has_ method instead (e.g., has_position()):

snapshot.position = [[0,0,0],[1,-1,1],[1.5,2.5,-3.5]]
snapshot.typeid[2] = 2
if not snapshot.has_mass():
    snapshot.mass = [2.,2.,10.]

Topology

The topology (bond information) can be stored in Bonds, Angles, Dihedrals, and Impropers objects. All these objects function similarly, differing only in the number of particles that are included in a connection (2 for a bond, 3 for an angle, 4 for a dihedral or improper). Each connection has an associated id and typeid.

bonds = Bonds(N=3, num_types=2)
angles = Angles(N=2, num_types=1)

These constructor arguments are available as attributes:

• N: number of connections (int)
• num_types: number of connection types (int). If num_types is None, then the number of types is deduced from typeid.

The data contained per connection is:

• num_members: (N, M) array of particles IDs in each topology (dtype: int, default: 1), where M is the number of particles in a connection.
• id: (N,) array topology IDs (dtype: int, default: runs from 1 to N)
• typeid: (N,) array of type indexes (dtype: int, default: 1)

All values of indexes will follow the LAMMPS 1-indexed convention, but the arrays themselves are 0-indexed. Lazy array initialization is used as for the Snapshot.

Data files

A LAMMPS data file is represented by a DataFile. The file must be explicitly read() to get a Snapshot:

f = lammpsio.DataFile("config.data")
snapshot = f.read()

The atom_style will be read from the comment in the Atoms section of the file. If it is not present, it must be specified in the DataFile. If atom_style is specified and also present in the file, the two must match or an error will be raised.

There are many sections that can be stored in a data file, but lammpsio does not currently understand all of them. You can check DataFile.known_headers, DataFile.unknown_headers, DataFile.known_bodies and DataFile.unknown_bodies for lists of what is currently supported.

A Snapshot can be written using the create() method:

f = lammpsio.DataFile.create("config2.data", snapshot)

A DataFile corresponding to the new file is returned by create().

Dump files

A LAMMPS dump file is represented by a DumpFile. The actual file format is very flexible, but by default embeds a schema that can be read:

traj = lammpsio.DumpFile(filename="atoms.lammpstrj")

If the schema does not exist for some reason, it can be manually specified as a dictionary. Valid keys for the schema match the names and shapes in the Snapshot. The keys requiring only 1 column index are: id, typeid, molecule, charge, and mass. The keys requiring 3 column indexes are position, velocity, and image.

LAMMPS will dump particles in an unknown order unless you have used the dump_modify sort option. If you want particles to be ordered by id in the Snapshot, use sort_ids=True (default).

A DumpFile is iterable, so you can use it to go through all the snapshots of a trajectory:

for snap in traj:
    print(snap.step)

You can also get the number of snapshots in the DumpFile, but this does require reading the entire file: so use with caution!

num_frames = len(traj)

Random access to snapshots is not currently implemented, but it may be added in future. If you want to randomly access snapshots, you should load the whole file into a list:

snaps = [snap for snap in traj]
print(snaps[3].step)

Keep in the mind that the memory requirements for this can be huge!

A DumpFile can be created from a list of snapshots:

t = lammpsio.DumpFile.create("atoms.lammpstrj", schema, snaps)

The object representing the new file is returned and can be used.