6.3. DCD trajectory I/O — MDAnalysis.coordinates.DCD

Classes to read and write DCD binary trajectories, the format used by CHARMM, NAMD, and also LAMMPS. Trajectories can be read regardless of system-endianness as this is auto-detected.

Generally, DCD trajectories produced by any code can be read (with the DCDReader) although there can be issues with the unitcell (simulation box) representation (see DCDReader.dimensions). DCDs can also be written but the DCDWriter follows recent NAMD/VMD convention for the unitcell but still writes AKMA time. Reading and writing these trajectories within MDAnalysis will work seamlessly but if you process those trajectories with other tools you might need to watch out that time and unitcell dimensions are correctly interpreted.

See also

MDAnalysis.coordinates.LAMMPS

module provides a more flexible DCD reader/writer.

6.3.1. Classes

class MDAnalysis.coordinates.DCD.DCDReader(filename, convert_units=True, dt=None, **kwargs)[source]

Reader for the DCD format.

DCD is used by NAMD, CHARMM and LAMMPS as the default trajectory format. The DCD file format is not well defined. In particular, NAMD and CHARMM use it differently. Currently, MDAnalysis tries to guess the correct format for the unitcell representation but it can be wrong. Check the unitcell dimensions, especially for triclinic unitcells (see Issue 187). DCD trajectories produced by CHARMM and NAMD( >2.5) record time in AKMA units. If other units have been recorded (e.g., ps) then employ the configurable :class:MDAnalysis.coordinates.LAMMPS.DCDReader and set the time unit as a optional argument. You can find a list of units used in the DCD formats on the MDAnalysis wiki.

MDAnalysis always uses (*A*, *B*, *C*, *alpha*, *beta*, *gamma*) to represent the unit cell. Lengths A, B, C are in the MDAnalysis length unit (Å), and angles are in degrees.

The ordering of the angles in the unitcell is the same as in recent versions of VMD’s DCDplugin (2013), namely the X-PLOR DCD format: The original unitcell is read as [A, gamma, B, beta, alpha, C] from the DCD file. If any of these values are < 0 or if any of the angles are > 180 degrees then it is assumed it is a new-style CHARMM unitcell (at least since c36b2) in which box vectors were recorded.

Deprecated since version 2.4.0: DCDReader currently makes independent timesteps by copying the Timestep associated with the reader. Other readers update the Timestep inplace meaning all references to the Timestep contain the same data. The unique independent Timestep behaviour of the DCDReader is deprecated will be changed in 3.0 to be the same as other readers

Warning

The DCD format is not well defined. Check your unit cell dimensions carefully, especially when using triclinic boxes. Different software packages implement different conventions and MDAnalysis is currently implementing the newer NAMD/VMD convention and tries to guess the new CHARMM one. Old CHARMM trajectories might give wrong unitcell values. For more details see Issue 187.

Parameters:

  • filename (str) – trajectory filename

  • convert_units (bool (optional)) – convert units to MDAnalysis units

  • dt (float (optional)) – overwrite time delta stored in DCD

  • **kwargs (dict) – General reader arguments.

Changed in version 0.17.0: Changed to use libdcd.pyx library and removed the correl function

OtherWriter(filename, **kwargs)

Returns a writer appropriate for filename.

Sets the default keywords start, step and dt (if available). n_atoms is always set from Reader.n_atoms.

See also

Reader.Writer()

Writer(filename, n_atoms=None, **kwargs)[source]

Return writer for trajectory format

add_auxiliary(aux_spec: str | Dict[str, str] | None = None, auxdata: str | AuxReader | None = None, format: str | None = None, **kwargs) → None

Add auxiliary data to be read alongside trajectory.

Auxiliary data may be any data timeseries from the trajectory additional to that read in by the trajectory reader. auxdata can be an AuxReader instance, or the data itself as e.g. a filename; in the latter case an appropriate AuxReader is guessed from the data/file format. An appropriate format may also be directly provided as a key word argument.

On adding, the AuxReader is initially matched to the current timestep of the trajectory, and will be updated when the trajectory timestep changes (through a call to next() or jumping timesteps with trajectory[i]).

The representative value(s) of the auxiliary data for each timestep (as calculated by the AuxReader) are stored in the current timestep in the ts.aux namespace under aux_spec; e.g. to add additional pull force data stored in pull-force.xvg:

u = MDAnalysis.Universe(PDB, XTC)
u.trajectory.add_auxiliary('pull', 'pull-force.xvg')

The representative value for the current timestep may then be accessed as u.trajectory.ts.aux.pull or u.trajectory.ts.aux['pull'].

The following applies to energy readers like the EDRReader.

All data that is present in the (energy) file can be added by omitting aux_spec like so:

u.trajectory.add_auxiliary(auxdata="ener.edr")

aux_spec is expected to be a dictionary that maps the desired attribute name in the ts.aux namespace to the precise data to be added as identified by a data_selector:

term_dict = {"temp": "Temperature", "epot": "Potential"}
u.trajectory.add_auxiliary(term_dict, "ener.edr")

Adding this data can be useful, for example, to filter trajectory frames based on non-coordinate data like the potential energy of each time step. Trajectory slicing allows working on a subset of frames:

selected_frames = np.array([ts.frame for ts in u.trajectory
                            if ts.aux.epot < some_threshold])
subset = u.trajectory[selected_frames]

See also

remove_auxiliary()

Note

Auxiliary data is assumed to be time-ordered, with no duplicates. See the Auxiliary API.

add_transformations(*transformations)

Add all transformations to be applied to the trajectory.

This function take as list of transformations as an argument. These transformations are functions that will be called by the Reader and given a Timestep object as argument, which will be transformed and returned to the Reader. The transformations can be part of the transformations module, or created by the user, and are stored as a list transformations. This list can only be modified once, and further calls of this function will raise an exception.

u = MDAnalysis.Universe(topology, coordinates)
workflow = [some_transform, another_transform, this_transform]
u.trajectory.add_transformations(*workflow)

The transformations are applied in the order given in the list transformations, i.e., the first transformation is the first or innermost one to be applied to the Timestep. The example above would be equivalent to

for ts in u.trajectory:
   ts = this_transform(another_transform(some_transform(ts)))
Parameters:

transform_list (list) – list of all the transformations that will be applied to the coordinates in the order given in the list

See also

MDAnalysis.transformations

property aux_list

Lists the names of added auxiliary data.

check_slice_indices(start, stop, step)

Check frame indices are valid and clip to fit trajectory.

The usage follows standard Python conventions for range() but see the warning below.

Parameters:

  • start (int or None) – Starting frame index (inclusive). None corresponds to the default of 0, i.e., the initial frame.

  • stop (int or None) – Last frame index (exclusive). None corresponds to the default of n_frames, i.e., it includes the last frame of the trajectory.

  • step (int or None) – step size of the slice, None corresponds to the default of 1, i.e, include every frame in the range start, stop.

Returns:

start, stop, step – Integers representing the slice

Return type:

tuple (int, int, int)

Warning

The returned values start, stop and step give the expected result when passed in range() but gives unexpected behavior when passed in a slice when stop=None and step=-1

This can be a problem for downstream processing of the output from this method. For example, slicing of trajectories is implemented by passing the values returned by check_slice_indices() to range()

range(start, stop, step)

and using them as the indices to randomly seek to. On the other hand, in MDAnalysis.analysis.base.AnalysisBase the values returned by check_slice_indices() are used to splice the trajectory by creating a slice instance

slice(start, stop, step)

This creates a discrepancy because these two lines are not equivalent:

range(10, -1, -1)             # [10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
range(10)[slice(10, -1, -1)]  # []
close()[source]

close reader

convert_forces_from_native(force, inplace=True)

Conversion of forces array force from native to base units

Parameters:

  • force (array_like) – Forces to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input force is modified in place and also returned. In-place operations improve performance because allocating new arrays is avoided.

New in version 0.7.7.

convert_forces_to_native(force, inplace=True)

Conversion of force array force from base to native units.

Parameters:

  • force (array_like) – Forces to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input force is modified in place and also returned. In-place operations improve performance because allocating new arrays is avoided.

New in version 0.7.7.

convert_pos_from_native(x, inplace=True)

Conversion of coordinate array x from native units to base units.

Parameters:

  • x (array_like) – Positions to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input x is modified in place and also returned. In-place operations improve performance because allocating new arrays is avoided.

Changed in version 0.7.5: Keyword inplace can be set to False so that a modified copy is returned unless no conversion takes place, in which case the reference to the unmodified x is returned.

convert_pos_to_native(x, inplace=True)

Conversion of coordinate array x from base units to native units.

Parameters:

  • x (array_like) – Positions to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input x is modified in place and also returned. In-place operations improve performance because allocating new arrays is avoided.

Changed in version 0.7.5: Keyword inplace can be set to False so that a modified copy is returned unless no conversion takes place, in which case the reference to the unmodified x is returned.

convert_time_from_native(t, inplace=True)

Convert time t from native units to base units.

Parameters:

  • t (array_like) – Time values to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input t is modified in place and also returned (although note that scalar values t are passed by value in Python and hence an in-place modification has no effect on the caller.) In-place operations improve performance because allocating new arrays is avoided.

Changed in version 0.7.5: Keyword inplace can be set to False so that a modified copy is returned unless no conversion takes place, in which case the reference to the unmodified x is returned.

convert_time_to_native(t, inplace=True)

Convert time t from base units to native units.

Parameters:

  • t (array_like) – Time values to transform

  • inplace (bool, optional) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input t is modified in place and also returned. (Also note that scalar values t are passed by value in Python and hence an in-place modification has no effect on the caller.)

Changed in version 0.7.5: Keyword inplace can be set to False so that a modified copy is returned unless no conversion takes place, in which case the reference to the unmodified x is returned.

convert_velocities_from_native(v, inplace=True)

Conversion of velocities array v from native to base units

Parameters:

  • v (array_like) – Velocities to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input v is modified in place and also returned. In-place operations improve performance because allocating new arrays is avoided.

New in version 0.7.5.

convert_velocities_to_native(v, inplace=True)

Conversion of coordinate array v from base to native units

Parameters:

  • v (array_like) – Velocities to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input v is modified in place and also returned. In-place operations improve performance because allocating new arrays is avoided.

New in version 0.7.5.

copy()

Return independent copy of this Reader.

New Reader will have its own file handle and can seek/iterate independently of the original.

Will also copy the current state of the Timestep held in the original Reader.

Changed in version 2.2.0: Arguments used to construct the reader are correctly captured and passed to the creation of the new class. Previously the only n_atoms was passed to class copies, leading to a class created with default parameters which may differ from the original class.

property dimensions

unitcell dimensions (A, B, C, alpha, beta, gamma)

property dt

timestep between frames

property frame: int

Frame number of the current time step.

This is a simple short cut to Timestep.frame.

get_aux_attribute(auxname, attrname)

Get the value of attrname from the auxiliary auxname

Parameters:

  • auxname (str) – Name of the auxiliary to get value for

  • attrname (str) – Name of gettable attribute in the auxiliary reader

See also

set_aux_attribute()

get_aux_descriptions(auxnames=None)

Get descriptions to allow reloading the specified auxiliaries.

If no auxnames are provided, defaults to the full list of added auxiliaries.

Passing the resultant description to add_auxiliary() will allow recreation of the auxiliary. e.g., to duplicate all auxiliaries into a second trajectory:

descriptions = trajectory_1.get_aux_descriptions()
for aux in descriptions:
    trajectory_2.add_auxiliary(**aux)
Returns:

List of dictionaries of the args/kwargs describing each auxiliary.

Return type:

list

See also

MDAnalysis.auxiliary.base.AuxReader.get_description()

iter_as_aux(auxname)

Iterate through timesteps for which there is at least one assigned step from the auxiliary auxname within the cutoff specified in auxname.

See also

next_as_aux(), iter_auxiliary()

iter_auxiliary(auxname, start=None, stop=None, step=None, selected=None)

Iterate through the auxiliary auxname independently of the trajectory.

Will iterate over the specified steps of the auxiliary (defaults to all steps). Allows to access all values in an auxiliary, including those out of the time range of the trajectory, without having to also iterate through the trajectory.

After interation, the auxiliary will be repositioned at the current step.

Parameters:

  • auxname (str) – Name of the auxiliary to iterate over.

  • (start (optional) – Options for iterating over a slice of the auxiliary.

  • stop (optional) – Options for iterating over a slice of the auxiliary.

  • step) (optional) – Options for iterating over a slice of the auxiliary.

  • selected (lst | ndarray, optional) – List of steps to iterate over.

Yields:

AuxStep object

See also

iter_as_aux()

property n_frames

number of frames in trajectory

next() → Timestep

Forward one step to next frame.

next_as_aux(auxname)

Move to the next timestep for which there is at least one step from the auxiliary auxname within the cutoff specified in auxname.

This allows progression through the trajectory without encountering NaN representative values (unless these are specifically part of the auxiliary data).

If the auxiliary cutoff is not set, where auxiliary steps are less frequent (auxiliary.dt > trajectory.dt), this allows progression at the auxiliary pace (rounded to nearest timestep); while if the auxiliary steps are more frequent, this will work the same as calling next().

See the Auxiliary API.

See also

iter_as_aux()

static parse_n_atoms(filename, **kwargs)[source]

Read the coordinate file and deduce the number of atoms

Returns:

n_atoms – the number of atoms in the coordinate file

Return type:

int

Raises:

NotImplementedError – when the number of atoms can’t be deduced

remove_auxiliary(auxname)

Clear data and close the AuxReader for the auxiliary auxname.

See also

add_auxiliary()

rename_aux(auxname, new)

Change the name of the auxiliary auxname to new.

Provided there is not already an auxiliary named new, the auxiliary name will be changed in ts.aux namespace, the trajectory’s list of added auxiliaries, and in the auxiliary reader itself.

Parameters:

  • auxname (str) – Name of the auxiliary to rename

  • new (str) – New name to try set

Raises:

ValueError – If the name new is already in use by an existing auxiliary.

rewind() → Timestep

Position at beginning of trajectory

set_aux_attribute(auxname, attrname, new)

Set the value of attrname in the auxiliary auxname.

Parameters:

  • auxname (str) – Name of the auxiliary to alter

  • attrname (str) – Name of settable attribute in the auxiliary reader

  • new – New value to try set attrname to

See also

get_aux_attribute(), rename_aux()

property time

Time of the current frame in MDAnalysis time units (typically ps).

This is either read straight from the Timestep, or calculated as time = Timestep.frame * Timestep.dt

timeseries(asel=None, atomgroup=None, start=None, stop=None, step=None, order='afc')[source]

Return a subset of coordinate data for an AtomGroup

Parameters:

  • asel (AtomGroup) –

    The AtomGroup to read the coordinates from. Defaults to None, in which case the full set of coordinate data is returned.

    Deprecated since version 2.7.0: asel argument will be renamed to atomgroup in 3.0.0

  • atomgroup (AtomGroup (optional)) – Same as asel, will replace asel in 3.0.0

  • start (int (optional)) – Begin reading the trajectory at frame index start (where 0 is the index of the first frame in the trajectory); the default None starts at the beginning.

  • stop (int (optional)) – End reading the trajectory at frame index stop-1, i.e, stop is excluded. The trajectory is read to the end with the default None.

  • step (int (optional)) – Step size for reading; the default None is equivalent to 1 and means to read every frame.

  • order (str (optional)) – the order/shape of the return data array, corresponding to (a)tom, (f)rame, (c)oordinates all six combinations of ‘a’, ‘f’, ‘c’ are allowed ie “fac” - return array where the shape is (frame, number of atoms, coordinates)

Changed in version 1.0.0: skip and format keywords have been removed.

Changed in version 2.4.0: ValueError now raised instead of NoDataError for empty input AtomGroup

property totaltime: float

Total length of the trajectory

The time is calculated as (n_frames - 1) * dt, i.e., we assume that the first frame no time as elapsed. Thus, a trajectory with two frames will be considered to have a length of a single time step dt and a “trajectory” with a single frame will be reported as length 0.

property transformations

Returns the list of transformations

units = {'length': 'Angstrom', 'time': 'AKMA'}

dict with units of of time and length (and velocity, force, … for formats that support it)

class MDAnalysis.coordinates.DCD.DCDWriter(filename, n_atoms, convert_units=True, step=1, dt=1, remarks='', nsavc=1, istart=0, **kwargs)[source]

DCD Writer class

The writer follows recent NAMD/VMD convention for the unitcell (box lengths in Å and angle-cosines, [A, cos(gamma), B, cos(beta), cos(alpha), C]) and writes positions in Å and time in AKMA time units.

Note

When writing out timesteps without dimensions (i.e. set None) the DCDWriter will write out a zeroed unitcell (i.e. [0, 0, 0, 0, 0, 0]). As this behaviour is poorly defined, it may not match the expectations of other software.

Parameters:

  • filename (str) – filename of trajectory

  • n_atoms (int) – number of atoms to be written

  • convert_units (bool (optional)) – convert from MDAnalysis units to format specific units

  • step (int (optional)) – number of steps between frames to be written

  • dt (float (optional)) – time between two frames. If None guess from first written TimeStep

  • remarks (str (optional)) – remarks to be stored in DCD. Shouldn’t be more then 240 characters

  • nsavc (int (optional)) – DCD files can also store the number of integrator time steps that correspond to the interval between two frames as nsavc (i.e., every how many MD steps is a frame saved to the DCD). By default, this number is just set to one and this should be sufficient for almost all cases but if required, nsavc can be changed.

  • istart (int (optional)) – starting frame number in integrator timesteps. CHARMM defaults to nsavc, i.e., start at frame number 1 = istart / nsavc. The value None will set istart to nsavc (the CHARMM default). The MDAnalysis default is 0 so that the frame number and time of the first frame is 0.

  • **kwargs (dict) – General writer arguments

close()[source]

close trajectory

convert_dimensions_to_unitcell(ts, inplace=True)

Read dimensions from timestep ts and return appropriate unitcell.

The default is to return [A,B,C,alpha,beta,gamma]; if this is not appropriate then this method has to be overriden.

convert_forces_from_native(force, inplace=True)

Conversion of forces array force from native to base units

Parameters:

  • force (array_like) – Forces to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input force is modified in place and also returned. In-place operations improve performance because allocating new arrays is avoided.

New in version 0.7.7.

convert_forces_to_native(force, inplace=True)

Conversion of force array force from base to native units.

Parameters:

  • force (array_like) – Forces to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input force is modified in place and also returned. In-place operations improve performance because allocating new arrays is avoided.

New in version 0.7.7.

convert_pos_from_native(x, inplace=True)

Conversion of coordinate array x from native units to base units.

Parameters:

  • x (array_like) – Positions to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input x is modified in place and also returned. In-place operations improve performance because allocating new arrays is avoided.

Changed in version 0.7.5: Keyword inplace can be set to False so that a modified copy is returned unless no conversion takes place, in which case the reference to the unmodified x is returned.

convert_pos_to_native(x, inplace=True)

Conversion of coordinate array x from base units to native units.

Parameters:

  • x (array_like) – Positions to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input x is modified in place and also returned. In-place operations improve performance because allocating new arrays is avoided.

Changed in version 0.7.5: Keyword inplace can be set to False so that a modified copy is returned unless no conversion takes place, in which case the reference to the unmodified x is returned.

convert_time_from_native(t, inplace=True)

Convert time t from native units to base units.

Parameters:

  • t (array_like) – Time values to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input t is modified in place and also returned (although note that scalar values t are passed by value in Python and hence an in-place modification has no effect on the caller.) In-place operations improve performance because allocating new arrays is avoided.

Changed in version 0.7.5: Keyword inplace can be set to False so that a modified copy is returned unless no conversion takes place, in which case the reference to the unmodified x is returned.

convert_time_to_native(t, inplace=True)

Convert time t from base units to native units.

Parameters:

  • t (array_like) – Time values to transform

  • inplace (bool, optional) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input t is modified in place and also returned. (Also note that scalar values t are passed by value in Python and hence an in-place modification has no effect on the caller.)

Changed in version 0.7.5: Keyword inplace can be set to False so that a modified copy is returned unless no conversion takes place, in which case the reference to the unmodified x is returned.

convert_velocities_from_native(v, inplace=True)

Conversion of velocities array v from native to base units

Parameters:

  • v (array_like) – Velocities to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input v is modified in place and also returned. In-place operations improve performance because allocating new arrays is avoided.

New in version 0.7.5.

convert_velocities_to_native(v, inplace=True)

Conversion of coordinate array v from base to native units

Parameters:

  • v (array_like) – Velocities to transform

  • inplace (bool (optional)) – Whether to modify the array inplace, overwriting previous data

Note

By default, the input v is modified in place and also returned. In-place operations improve performance because allocating new arrays is avoided.

New in version 0.7.5.

has_valid_coordinates(criteria, x)

Returns True if all values are within limit values of their formats.

Due to rounding, the test is asymmetric (and min is supposed to be negative):

min < x <= max
Parameters:

  • criteria (dict) – dictionary containing the max and min values in native units

  • x (numpy.ndarray) – (x, y, z) coordinates of atoms selected to be written out

Return type:

bool

units = {'length': 'Angstrom', 'time': 'AKMA'}

dict with units of of time and length (and velocity, force, … for formats that support it)

write(obj)

Write current timestep, using the supplied obj.

Parameters:

obj (AtomGroup or Universe) – write coordinate information associate with obj

Note

The size of the obj must be the same as the number of atoms provided when setting up the trajectory.

Changed in version 2.0.0: Deprecated support for Timestep argument to write has now been removed. Use AtomGroup or Universe as an input instead.