11.2.1. Core object: Universe — MDAnalysis.core.universe

The Universe class ties a topology and a trajectory together. Almost all code in MDAnalysis starts with a Universe.

Normally, a Universe is created from files:

import MDAnalysis as mda
u = mda.Universe("topology.psf", "trajectory.dcd")

In order to construct new simulation system it is also convenient to construct a Universe from existing AtomGroup instances with the Merge() function.

11.2.1.1. Classes

class MDAnalysis.core.universe.Universe(topology=None, *coordinates, all_coordinates=False, format=None, topology_format=None, transformations=None, guess_bonds=False, vdwradii=None, fudge_factor=0.55, lower_bound=0.1, in_memory=False, in_memory_step=1, **kwargs)[source]

The MDAnalysis Universe contains all the information describing the system.

The system always requires a topology file — in the simplest case just a list of atoms. This can be a CHARMM/NAMD PSF file or a simple coordinate file with atom informations such as XYZ, PDB, GROMACS GRO or TPR, or CHARMM CRD. See Table of Supported Topology Formats for what kind of topologies can be read.

A trajectory file provides coordinates; the coordinates have to be ordered in the same way as the list of atoms in the topology. A trajectory can be a single frame such as a PDB, CRD, or GRO file, or it can be a MD trajectory (in CHARMM/NAMD/LAMMPS DCD, GROMACS XTC/TRR, AMBER nc, generic XYZ format, …). See Table of supported coordinate formats for what can be read as a “trajectory”.

As a special case, when the topology is a file that contains atom information and coordinates (such as XYZ, PDB, GRO or CRD, see Table of supported coordinate formats) then the coordinates are immediately loaded from the “topology” file unless a trajectory is supplied.

Parameters

  • topology (str, stream, Topology, numpy.ndarray, None) – A CHARMM/XPLOR PSF topology file, PDB file or Gromacs GRO file; used to define the list of atoms. If the file includes bond information, partial charges, atom masses, … then these data will be available to MDAnalysis. Alternatively, an existing MDAnalysis.core.topology.Topology instance may be given, numpy coordinates, or None for an empty universe.

  • coordinates (str, stream, list of str, list of stream (optional)) – Coordinates can be provided as files of a single frame (eg a PDB, CRD, or GRO file); a list of single frames; or a trajectory file (in CHARMM/NAMD/LAMMPS DCD, Gromacs XTC/TRR, or generic XYZ format). The coordinates must be ordered in the same way as the list of atoms in the topology. See Table of supported coordinate formats for what can be read as coordinates. Alternatively, streams can be given.

  • topology_format (str, None, default None) – Provide the file format of the topology file; None guesses it from the file extension. Can also pass a subclass of MDAnalysis.topology.base.TopologyReaderBase to define a custom reader to be used on the topology file.

  • format (str, None, default None) – Provide the file format of the coordinate or trajectory file; None guesses it from the file extension. Note that this keyword has no effect if a list of file names is supplied because the “chained” reader has to guess the file format for each individual list member. Can also pass a subclass of MDAnalysis.coordinates.base.ProtoReader to define a custom reader to be used on the trajectory file.

  • all_coordinates (bool, default False) – If set to True specifies that if more than one filename is passed they are all to be used, if possible, as coordinate files (employing a MDAnalysis.coordinates.chain.ChainReader). The default behavior is to take the first file as a topology and the remaining as coordinates. The first argument will always always be used to infer a topology regardless of all_coordinates.

  • guess_bonds (bool, default False) – Once Universe has been loaded, attempt to guess the connectivity between atoms. This will populate the .bonds, .angles, and .dihedrals attributes of the Universe.

  • vdwradii (dict, None, default None) – For use with guess_bonds. Supply a dict giving a vdwradii for each atom type which are used in guessing bonds.

  • fudge_factor (float, default [0.55]) – For use with guess_bonds. Supply the factor by which atoms must overlap each other to be considered a bond.

  • lower_bound (float, default [0.1]) – For use with guess_bonds. Supply the minimum bond length.

  • transformations (function or list, None, default None) – Provide a list of transformations that you wish to apply to the trajectory upon reading. Transformations can be found in MDAnalysis.transformations, or can be user-created.

  • in_memory (bool, default False) – After reading in the trajectory, transfer it to an in-memory representations, which allow for manipulation of coordinates.

  • in_memory_step (int, default 1) – Only read every nth frame into in-memory representation.

  • continuous (bool, default False) – The continuous option is used by the ChainReader, which contains the functionality to treat independent trajectory files as a single virtual trajectory.

  • **kwargs (extra arguments are passed to the topology parser.) –

trajectory

currently loaded trajectory reader; readers are described in Coordinates modules

Type

base.ReaderBase or base.SingleFrameReaderBase

dimensions

system dimensions (simulation unit cell, if set in the trajectory) at the current time step (see MDAnalysis.coordinates.timestep.Timestep.dimensions). The unit cell can be set for the current time step (but the change is not permanent unless written to a file).

Type

numpy.ndarray

atoms

all particles (Atom) in the system, as read from the topology file

Type

AtomGroup

residues

all residues (Residue) in the system

Type

ResidueGroup

segments

all segments (Segment) in the system

Type

SegmentGroup

bonds

all bonds (if defined in the topology) as provided by Universe.atoms.bonds

Type

topologyattrs.Bonds

angles

all angles (if defined in the topology), same as Universe.atoms.angles

Type

topologyattrs.Angles

dihedrals

all dihedral angles (if defined in the topology), same as Universe.atoms.dihedrals

Type

topologyattrs.Dihedrals

impropers

all improper dihedral angles (if defined in the topology), same as Universe.atoms.impropers

Type

topologyattrs.Impropers

Examples

Examples for setting up a Universe:

u = Universe(topology, trajectory)          # read system from file(s)
u = Universe(pdbfile)                       # read atoms and coordinates from PDB or GRO
u = Universe(topology, [traj1, traj2, ...]) # read from a list of trajectories
u = Universe(topology, traj1, traj2, ...)   # read from multiple trajectories

Load new data into a universe (replaces old trajectory and does not append):

u.load_new(trajectory)                      # read from a new trajectory file

Selecting atoms with select_atoms()

ag = u.select_atoms(...)

returns an AtomGroup.

Changed in version 1.0.0: Universe() now raises an error. Use Universe(None) or Universe.empty() instead. Removed instant selectors.

Changed in version 2.0.0: Universe now can be (un)pickled. topology and trajectory are reserved upon unpickle.

Changed in version 2.5.0: Added fudge_factor and lower_bound parameters for use with guess_bonds.

add_Residue(segment=None, **attrs)[source]

Add a new Residue to this Universe

New Residues will not contain any Atoms, but can be assigned to Atoms as per usual. If the Universe contains multiple segments, this must be specified as a keyword.

Parameters

  • segment (MDAnalysis.Segment) – If there are multiple segments, then the Segment that the new Residue will belong in must be specified.

  • attrs (dict) – For each Residue attribute, the value for the new Residue must be specified

Return type

A reference to the new Residue

Raises

NoDataError – If any information was missing. This happens before any changes have been made, ie the change is rolled back.

Example

Adding a new GLY residue, then placing atoms within it:

>>> import MDAnalysis as mda
>>> from MDAnalysis.tests.datafiles import PSF, DCD
>>> u = mda.Universe(PSF, DCD)
>>> newres = u.add_Residue(segment=u.segments[0], resid=42, resname='GLY', resnum=0)
>>> u.atoms[[1, 2, 3]].residues = newres
>>> u.select_atoms('resname GLY and resid 42 and resnum 0')
<AtomGroup with 3 atoms>
add_Segment(**attrs)[source]

Add a new Segment to this Universe

Parameters

attrs (dict) – For each Segment attribute as a key, give the value in the new Segment

Return type

A reference to the new Segment

Raises

NoDataError – If any attributes were not specified as a keyword.

add_TopologyAttr(topologyattr, values=None)[source]

Add a new topology attribute to the Universe

Adding a TopologyAttribute to the Universe makes it available to all AtomGroups etc throughout the Universe.

Parameters

  • topologyattr (TopologyAttr or string) – Either a MDAnalysis TopologyAttr object or the name of a possible topology attribute.

  • values (np.ndarray, optional) – If initiating an attribute from a string, the initial values to use. If not supplied, the new TopologyAttribute will have empty or zero values.

Example

For example to add bfactors to a Universe:

>>> import MDAnalysis as mda
>>> from MDAnalysis.tests.datafiles import PSF, DCD
>>> u = mda.Universe(PSF, DCD)
>>> u.add_TopologyAttr('tempfactors')
>>> u.atoms.tempfactors
array([0., 0., 0., ..., 0., 0., 0.])

Changed in version 0.17.0: Can now also add TopologyAttrs with a string of the name of the attribute to add (eg ‘charges’), can also supply initial values using values keyword.

Changed in version 1.1.0: Now warns when adding bfactors to a Universe with existing tempfactors, or adding tempfactors to a Universe with existing bfactors. In version 2.0, MDAnalysis will stop treating tempfactors and bfactors as separate attributes. Instead, they will be aliases of the same attribute.

add_angles(values, types=None, guessed=False)[source]

Add new Angles to this Universe.

Parameters

  • values (iterable of tuples, AtomGroups, or Angles; or TopologyGroup) – An iterable of: tuples of 3 atom indices, or AtomGroups with 3 atoms, or Angles. If every value is a Angle, all keywords are ignored. If AtomGroups, Angles, or a TopologyGroup are passed, they must be from the same Universe.

  • types (iterable (optional, default None)) – None, or an iterable of hashable values with the same length as values

  • guessed (bool or iterable (optional, default False)) – bool, or an iterable of hashable values with the same length as values

  • versionadded: (..) – 1.0.0:

add_bonds(values, types=None, guessed=False, order=None)[source]

Add new Bonds to this Universe.

Parameters

  • values (iterable of tuples, AtomGroups, or Bonds; or TopologyGroup) – An iterable of: tuples of 2 atom indices, or AtomGroups with 2 atoms, or Bonds. If every value is a Bond, all keywords are ignored. If AtomGroups, Bonds, or a TopologyGroup are passed, they must be from the same Universe.

  • types (iterable (optional, default None)) – None, or an iterable of hashable values with the same length as values

  • guessed (bool or iterable (optional, default False)) – bool, or an iterable of hashable values with the same length as values

  • order (iterable (optional, default None)) – None, or an iterable of hashable values with the same length as values

Example

Adding TIP4P water bonds with a list of AtomGroups:

import MDAnalysis as mda
from MDAnalysis.tests.datafiles import GRO
u = mda.Universe(GRO)
sol = u.select_atoms('resname SOL')
ow_hw1 = sol.select_atoms('name OW or name HW1').split('residue')
ow_hw2 = sol.select_atoms('name OW or name HW2').split('residue')
ow_mw = sol.select_atoms('name OW or name MW').split('residue')
u.add_bonds(ow_hw1 + ow_hw2 + ow_mw)

You can only add bonds from the same Universe. If you would like to add AtomGroups, Bonds, or a TopologyGroup from a different Universe, convert them to indices first.

from MDAnalysis.tests.datafiles import PSF
u2 = mda.Universe(PSF)

#  assuming you have already added bonds to u
u2.add_bonds(u.bonds.to_indices())

New in version 1.0.0.

add_dihedrals(values, types=None, guessed=False)[source]

Add new Dihedrals to this Universe.

Parameters

  • values (iterable of tuples, AtomGroups, or Dihedrals; or TopologyGroup) – An iterable of: tuples of 4 atom indices, or AtomGroups with 4 atoms, or Dihedrals. If every value is a Dihedral, all keywords are ignored. If AtomGroups, Dihedrals, or a TopologyGroup are passed, they must be from the same Universe.

  • types (iterable (optional, default None)) – None, or an iterable of hashable values with the same length as values

  • guessed (bool or iterable (optional, default False)) – bool, or an iterable of hashable values with the same length as values

New in version 1.0.0.

add_impropers(values, types=None, guessed=False)[source]

Add new Impropers to this Universe.

Parameters

  • values (iterable of tuples, AtomGroups, or Impropers; or TopologyGroup) – An iterable of: tuples of 4 atom indices, or AtomGroups with 4 atoms, or Impropers. If every value is an Improper, all keywords are ignored. If AtomGroups, Impropers, or a TopologyGroup are passed, they must be from the same Universe.

  • types (iterable (optional, default None)) – None, or an iterable of hashable values with the same length as values

  • guessed (bool or iterable (optional, default False)) – bool, or an iterable of hashable values with the same length as values

New in version 1.0.0.

property coord

Reference to current timestep and coordinates of universe.

The raw trajectory coordinates are Universe.coord.positions, represented as a numpy.float32 array.

Because coord is a reference to a Timestep, it changes its contents while one is stepping through the trajectory.

Note

In order to access the coordinates it is better to use the AtomGroup.positions() method; for instance, all coordinates of the Universe as a numpy array: Universe.atoms.positions().

copy()[source]

Return an independent copy of this Universe

del_TopologyAttr(topologyattr)[source]

Remove a topology attribute from the Universe

Removing a TopologyAttribute from the Universe makes it unavailable to all AtomGroups etc throughout the Universe.

Parameters

topologyattr (TopologyAttr or string) – Either a MDAnalysis TopologyAttr object or the name of a possible topology attribute.

Example

For example to remove bfactors to a Universe:

>>> import MDAnalysis as mda
>>> from MDAnalysis.tests.datafiles import PSF, DCD
>>> u = mda.Universe(PSF, DCD)
>>> u.add_TopologyAttr('tempfactors')
>>> hasattr(u.atoms[:3], 'tempfactors')
True
>>>
>>> u.del_TopologyAttr('tempfactors')
>>> hasattr(u.atoms[:3], 'tempfactors')
False

New in version 2.0.0.

delete_angles(values)[source]

Delete Angles from this Universe.

Parameters

values (iterable of tuples, AtomGroups, or Angles; or TopologyGroup) – An iterable of: tuples of 3 atom indices, or AtomGroups with 3 atoms, or Angles. If AtomGroups, Angles, or a TopologyGroup are passed, they must be from the same Universe.

New in version 1.0.0.

delete_bonds(values)[source]

Delete Bonds from this Universe.

Parameters

values (iterable of tuples, AtomGroups, or Bonds; or TopologyGroup) – An iterable of: tuples of 2 atom indices, or AtomGroups with 2 atoms, or Bonds. If AtomGroups, Bonds, or a TopologyGroup are passed, they must be from the same Universe.

Example

Deleting bonds from a Universe:

import MDAnalysis as mda
from MDAnalysis.tests.datafiles import PSF
u = mda.Universe(PSF)

#  delete first 5 bonds
u.delete_bonds(u.bonds[:5])

If you are deleting bonds in the form of AtomGroups, Bonds, or a TopologyGroup, they must come from the same Universe. If you want to delete bonds from another Universe, convert them to indices first.

from MDAnalysis.tests.datafiles import PDB
u2 = mda.Universe(PDB)

u.delete_bonds(u2.bonds.to_indices())

New in version 1.0.0.

delete_dihedrals(values)[source]

Delete Dihedrals from this Universe.

Parameters

values (iterable of tuples, AtomGroups, or Dihedrals; or TopologyGroup) – An iterable of: tuples of 4 atom indices, or AtomGroups with 4 atoms, or Dihedrals. If AtomGroups, Dihedrals, or a TopologyGroup are passed, they must be from the same Universe.

New in version 1.0.0.

delete_impropers(values)[source]

Delete Impropers from this Universe.

Parameters

values (iterable of tuples, AtomGroups, or Impropers; or TopologyGroup) – An iterable of: tuples of 4 atom indices, or AtomGroups with 4 atoms, or Impropers. If AtomGroups, Angles, or a TopologyGroup are passed, they must be from the same Universe.

New in version 1.0.0.

classmethod empty(n_atoms, n_residues=1, n_segments=1, n_frames=1, atom_resindex=None, residue_segindex=None, trajectory=False, velocities=False, forces=False)[source]

Create a blank Universe

Useful for building a Universe without requiring existing files, for example for system building.

If trajectory is set to True, a MDAnalysis.coordinates.memory.MemoryReader will be attached to the Universe.

Parameters

  • n_atoms (int) – number of Atoms in the Universe

  • n_residues (int, default 1) – number of Residues in the Universe, defaults to 1

  • n_segments (int, default 1) – number of Segments in the Universe, defaults to 1

  • n_frames (int, default 1) – number of Frames in the Universe, defaults to 1

  • atom_resindex (array like, optional) – mapping of atoms to residues, e.g. with 6 atoms, atom_resindex=[0, 0, 1, 1, 2, 2] would put 2 atoms into each of 3 residues.

  • residue_segindex (array like, optional) – mapping of residues to segments

  • trajectory (bool, optional) – if True, attaches a MDAnalysis.coordinates.memory.MemoryReader allowing coordinates to be set and written.

  • velocities (bool, optional) – include velocities in the MDAnalysis.coordinates.memory.MemoryReader

  • forces (bool, optional) – include forces in the MDAnalysis.coordinates.memory.MemoryReader

Returns

Universe instance with dummy values for atoms and undefined coordinates/velocities/forces

Return type

Universe

Examples

For example to create a new Universe with 6 atoms in 2 residues, with positions for the atoms and a mass attribute:

u = mda.Universe.empty(6, 2,
                       atom_resindex=np.array([0, 0, 0, 1, 1, 1]),
                       trajectory=True,
      )
u.add_TopologyAttr('masses')

New in version 0.17.0.

Changed in version 0.19.0: The attached Reader when trajectory=True is now a MemoryReader

Changed in version 1.0.0: Universes can now be created with 0 atoms

classmethod from_smiles(smiles, sanitize=True, addHs=True, generate_coordinates=True, numConfs=1, rdkit_kwargs={}, **kwargs)[source]

Create a Universe from a SMILES string with rdkit

Parameters

  • smiles (str) – SMILES string

  • sanitize (bool (optional, default True)) – Toggle the sanitization of the molecule

  • addHs (bool (optional, default True)) – Add all necessary hydrogens to the molecule

  • generate_coordinates (bool (optional, default True)) – Generate 3D coordinates using RDKit’s AllChem.EmbedMultipleConfs() function. Requires adding hydrogens with the addHs parameter

  • numConfs (int (optional, default 1)) – Number of frames to generate coordinates for. Ignored if generate_coordinates=False

  • rdkit_kwargs (dict (optional)) – Other arguments passed to the RDKit EmbedMultipleConfs() function

  • kwargs (dict) – Parameters passed on Universe creation

Returns

universe – contains atom names and topology information (bonds) derived from the input SMILES string; coordinates are included if generate_coordinates was set to True

Return type

Universe

Examples

To create a Universe with 10 conformers of ethanol:

>>> from rdkit.Chem import AllChem
>>> u = mda.Universe.from_smiles('CCO', numConfs=10)
>>> u
<Universe with 9 atoms>
>>> u.trajectory
<RDKitReader with 10 frames of 9 atoms>

To use a different conformer generation algorithm, like ETKDGv3:

>>> u = mda.Universe.from_smiles('CCO', rdkit_kwargs=dict(
...      params=AllChem.ETKDGv3()))
>>> u.trajectory
<RDKitReader with 1 frames of 9 atoms>

New in version 2.0.0.

property kwargs

keyword arguments used to initialize this universe

load_new(filename, format=None, in_memory=False, in_memory_step=1, **kwargs)[source]

Load coordinates from filename.

The file format of filename is autodetected from the file name suffix or can be explicitly set with the format keyword. A sequence of files can be read as a single virtual trajectory by providing a list of filenames.

Parameters

  • filename (str or list) – the coordinate file (single frame or trajectory) or a list of filenames, which are read one after another.

  • format (str or list or object (optional)) – provide the file format of the coordinate or trajectory file; None guesses it from the file extension. Note that this keyword has no effect if a list of file names is supplied because the “chained” reader has to guess the file format for each individual list member [None]. Can also pass a subclass of MDAnalysis.coordinates.base.ProtoReader to define a custom reader to be used on the trajectory file.

  • in_memory (bool (optional)) –

    Directly load trajectory into memory with the MemoryReader

    New in version 0.16.0.

  • **kwargs (dict) – Other kwargs are passed to the trajectory reader (only for advanced use)

Returns

universe

Return type

Universe

Raises

TypeError – if trajectory format can not be determined or no appropriate trajectory reader found

Changed in version 0.8: If a list or sequence that is provided for filename only contains a single entry then it is treated as single coordinate file. This has the consequence that it is not read by the ChainReader but directly by its specialized file format reader, which typically has more features than the ChainReader.

Changed in version 0.17.0: Now returns a Universe instead of the tuple of file/array and detected file type.

Changed in version 2.4.0: Passes through kwargs if in_memory=True.

property models

Models in this Universe.

The MMTF format can define various models for a given structure. The topology (eg residue identity) can change between different models, resulting in a different number of atoms in each model.

Return type

A list of AtomGroups, each representing a single model.

Note

This requires the underlying topology to have models. Otherwise, a NoDataError is raised.

select_atoms(*args, **kwargs)[source]

Select atoms.

See also

MDAnalysis.core.groups.AtomGroup.select_atoms()

transfer_to_memory(start=None, stop=None, step=None, verbose=False, **kwargs)[source]

Transfer the trajectory to in memory representation.

Replaces the current trajectory reader object with one of type MDAnalysis.coordinates.memory.MemoryReader to support in-place editing of coordinates.

Parameters

  • start (int, optional) – start reading from the nth frame.

  • stop (int, optional) – read upto and excluding the nth frame.

  • step (int, optional) – Read in every nth frame. [1]

  • verbose (bool, optional) – Will print the progress of loading trajectory to memory, if set to True. Default value is False.

New in version 0.16.0.

Changed in version 2.4.0: Passes through kwargs to MemoryReader

11.2.1.2. Functions

MDAnalysis.core.universe.Merge(*args)[source]

Create a new new Universe from one or more AtomGroup instances.

Parameters

*args (AtomGroup) – One or more AtomGroups.

Returns

universe

Return type

Universe

Raises

  • ValueError – Too few arguments or an AtomGroup is empty and

  • TypeError – Arguments are not AtomGroup instances.

Notes

The resulting Universe will only inherit the common topology attributes that all merged universes share.

AtomGroup instances can come from different Universes, or can come directly from a select_atoms() call.

Merge can also be used with a single AtomGroup if the user wants to, for example, re-order the atoms in the Universe.

If multiple AtomGroup instances from the same Universe are given, the merge will first simply “add” together the AtomGroup instances.

Merging does not create a full trajectory but only a single structure even if the input consists of one or more trajectories. However, one can use the MemoryReader to construct a trajectory for the new Universe as described under Creating an in-memory trajectory of a sub-system.

Example

In this example, protein, ligand, and solvent were externally prepared in three different PDB files. They are loaded into separate Universe objects (where they could be further manipulated, e.g. renumbered, relabeled, rotated, …) The Merge() command is used to combine all of them together:

u1 = Universe("protein.pdb")
u2 = Universe("ligand.pdb")
u3 = Universe("solvent.pdb")
u = Merge(u1.select_atoms("protein"), u2.atoms, u3.atoms)
u.atoms.write("system.pdb")

The complete system is then written out to a new PDB file.

Changed in version 0.9.0: Raises exceptions instead of assertion errors.

Changed in version 0.16.0: The trajectory is now a MemoryReader.