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1. Overview over MDAnalysis

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3. Analysis modules

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2. Selection Commands

Once you have the Universe() object, you can select atoms (using a syntax very similar to CHARMM’s atom selection syntax):

>>> kalp = universe.selectAtoms("segid KALP")

MDAnalysis.core.Universe.selectAtoms() returns a MDAnalysis.core.AtomGroup.AtomGroup, so you can use all the methods defined for AtomGroups on them. Selections always return an AtomGroup with atoms sorted according to their index in the topology (this is to ensure that there aren’t any duplicates, which can happen with complicated selections).

One can group subselections using parentheses:

>>> universe.selectAtoms("segid DMPC and not (name H* or name O*)")
<AtomGroup with 3420 atoms>

Almost all the basic CHARMM selections work.

It is also possible to export selections for external software packages with the help of Selection exporters.

2.1. Selection Keywords

The following describes all selection keywords currently understood by the selection parser. The following applies to all selections:

  • Keywords are case sensitive.
  • Atoms are automatically sequentially ordered in a resulting selection (see notes below on Ordered selections for how to circumvent this if necessary).
  • Selections are parsed left to right and parentheses can be used for grouping.
  • Currently, only “stemming” is implemented as a primitive form of pattern matching: Using the * character in a string such as GL* selects all strings that start with “GL” such as “GLU”, “GLY”, “GLX29”, “GLN”.

2.1.1. Simple selections

protein, backbone, nucleic, nucleicbackbone
selects all atoms that belong to a standard set of residues; a protein is identfied by a hard-coded set of residue names so it may not work for esoteric residues.
segid seg-name
select by segid (as given in the topology), e.g. segid 4AKE or segid DMPC
resid residue-number-range
resid can take a single residue number or a range of numbers. A range consists of two numbers separated by a colon (inclusive) such as resid 1:5. A residue number (“resid”) is taken directly from the topology.
resnum resnum-number-range
resnum is the canonical residue number; typically it is set to the residue id in the original PDB structure.
resname residue-name
select by residue name, e.g. resname LYS
name atom-name
select by atom name (as given in the topology). Often, this is force field dependent. Example: name CA (for C&alpha; atoms) or name OW (for SPC water oxygen)
type atom-type
select by atom type; this is either a string or a number and depends on the force field; it is read from the topology file (e.g. the CHARMM PSF file contains numeric atom types). It has non-sensical values when a PDB or GRO file is used as a topology.
atom seg-name residue-number atom-name
a selector for a single atom consisting of segid resid atomname, e.g. DMPC 1 C2 selects the C2 carbon of the first residue of the DMPC segment

2.1.2. Boolean

all atoms not in the selection, e.g. not protein selects all atoms that aren’t part of a protein
and, or
combine two selections according to the rules of boolean algebra, e.g. protein and not (resname ALA or resname LYS) selects all atoms that belong to a protein, but are not in a lysine or alanine residue

2.1.3. Geometric

around distance selection
selects all atoms a certain cutoff away from another selection, e.g. around 3.5 protein selects all atoms not belonging to protein that are within 3.5 Angstroms from the protein
point x y z distance
selects all atoms within a cutoff of a point in space, make sure coordinate is separated by spaces, e.g. point 5.0 5.0 5.0  3.5 selects all atoms within 3.5 Angstroms of the coordinate (5.0, 5.0, 5.0)
prop [abs] property operator value
selects atoms based on position, using property x, y, or z coordinate. Supports the abs keyword (for absolute value) and the following operators: <, >, <=, >=, ==, !=. For example, prop z >= 5.0 selects all atoms with z coordinate greater than 5.0; prop abs z <= 5.0 selects all atoms within -5.0 <= z <= 5.0.

From version 0.6 onwards, geometric selections can use a k-d tree based, fast search algorithm (about three times faster than the previous version). However, it does not take periodicity into account. The fast algorithm is the default for around. Periodicity is only taken into account with the distance_array() functions via a minimum image convention (and this only works for rectangular simulation cells). If periodic boundary conditions should be taken into account then change the default behaviour of MDAnalysi by setting these two flags:

MDAnalysis.core.flags['use_periodic_selections'] = True
MDAnalysis.core.flags['use_KDTree_routines'] = False

2.1.4. Connectivity

byres selection
selects all atoms that are in the same segment and residue as selection, e.g. specify the subselection after the byres keyword

2.1.5. Index

bynum index-range
selects all atoms within a range of (1-based) inclusive indices, e.g. bynum 1 selects the first atom in the universe; bynum 5:10 selects atoms 5 through 10 inclusive. All atoms in the MDAnalysis.Universe are consecutively numbered, and the index runs from 1 up to the total number of atoms.

2.2. Instant selectors

For interactive work it becomes rather tedious to type common selection strings repeatedly. MDAnalysis automatically generates a number of instant selectors as attributes of the Universe and number of other levels of the structural hierarchy, namely for AtomGroup, Residue, ResidueGroup, Segment and SegmentGroup.

2.2.1. Segment selector

  • universe.<segid> or universe.s<segid> (if <segid> starts with a number)

  • returns a Segment

  • works for Universe and SegmentGroup

  • example
    >>> u.s4AKE
    <Segment '4AKE'>

2.2.2. Resid selector

  • seg.r<N> selects residue with number <N>

  • returns a Residue

  • works for Segment and SegmentGroup

  • example
    >>>  u.s4AKE.r100
    <Residue 'GLY', 100>

2.2.3. Residue name selector

  • seg.<resname> selects residues with residue name <resname>

  • returns a ResidueGroup

  • works for Segment and SegmentGroup

  • examples
    >>> u.s4AKE.MET
    <ResidueGroup [<Residue 'MET', 1>, <Residue 'MET', 21>, <Residue 'MET', 34>, <Residue 'MET', 53>, <Residue 'MET', 96>, <Residue 'MET', 174>]>
    >>> u.s4AKE.CYS
    <ResidueGroup [<Residue 'CYS', 77>]>
    >>> u.s4AKE.TRP
    NoDataError: No atoms defined for AtomGroup
  • The result is always a ResidueGroup; if no residues can be found then a MDAnalysis.NoDataError is raised.

2.2.4. Atom name selector

  • g.<atomname> selects a single atom or a group of atoms with name <atomname>

  • returns
    • a Atom if only a single atom was found,
    • a AtomGroup if more than one atom was found, or
    • raises a MDAnalysis.SelectionError if no atom was found.
  • works for any group derived from AtomGroup (i.e. all the ones mentioned above)

  • examples
    >>> u.atoms.CG
    >>> <AtomGroup with 125 atoms>
    >>> u.s4AKE.CG
    <AtomGroup with 125 atoms>
    >>> u.s4AKE.r100.CA
    < Atom 1516: name 'CA' of type '23' of resname 'GLY', resid 100 and segid '4AKE'>
    >>> u.s4AKE.r100.CB
    SelectionError: No atom in residue GLY with name CB

2.3. Ordered selections

selectAtoms() sorts the atoms in the AtomGroup by atom index before returning them (this is to eliminate possible duplicates in the selection). If the ordering of atoms is crucial (for instance when describing angles or dihedrals) or if duplicate atoms are required then one has to concatenate multiple AtomGroups, which does not sort them.

The most straightforward way to concatentate two AtomGroups is by using the + operator:

>>> ordered = u.selectAtoms("segid DMPC and resid 3 and name P") + u.selectAtoms("segid DMPC and resid 2 and name P")
>>> print list(ordered)
[< Atom 570: name 'P' of type '180' of resid 'DMPC', 3 and 'DMPC'>,
< Atom 452: name 'P' of type '180' of resid 'DMPC', 2 and 'DMPC'>]

A shortcut is to provide two or more selections to selectAtoms(), which then does the concatenation automatically:

>>> print list(universe.selectAtoms("segid DMPC and resid 3 and name P", "segid DMPC and resid 2 and name P"))
[< Atom 570: name 'P' of type '180' of resid 'DMPC', 3 and 'DMPC'>,
< Atom 452: name 'P' of type '180' of resid 'DMPC', 2 and 'DMPC'>]

Just for comparison to show that a single selection string does not work as one might expect:

>>> print list(universe.selectAtoms("segid DMPC and ( resid 3 or resid 2 ) and name P"))
[< Atom 452: name 'P' of type '180' of resid 'DMPC', 2 and 'DMPC'>,
< Atom 570: name 'P' of type '180' of resid 'DMPC', 3 and 'DMPC'>]