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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

> not
>   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:

    # WRONG!
    >>> 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'>]

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