# 13.3.1. Low-level Gromacs XDR trajectory reading — MDAnalysis.lib.formats.libmdaxdr¶

libmdaxdr contains the classes XTCFile and TRRFile. Both can be used to read and write frames from and to Gromacs XTC and TRR files. These classes are used internally by MDAnalysis in MDAnalysis.coordinates.XTC and MDAnalysis.coordinates.TRR. They behave similar to normal file objects.

For example, one can use a XTCFile to directly calculate mean coordinates (where the coordinates are stored in x attribute of the namedtuple frame):

with XTCFile("trajectory.xtc") as xtc:
n_atoms = xtc.n_atoms
mean = np.zeros((n_atoms, 3))
# iterate over trajectory
for frame in xtc:
mean += frame.x


The XTCFile class can be useful as a compressed storage format.

Besides iteration, XTCFile and TRRFile one can also seek to arbitrary frames using the seek() method. This is provided by lazily generating a offset list for stored frames. The offset list is generated the first time len() or :~XTCFile.seek is called.

(For more details on how to use XTCFile and TRRFile on their own please see the source code in lib/formats/libmdaxdr.pyx for the time being.)

class MDAnalysis.lib.formats.libmdaxdr.TRRFile

File-like wrapper for gromacs TRR files.

This class can be similar to the normal file objects in python. The read() function will return a frame and all information in it instead of a single line. Additionally the context-manager protocoll is supported as well.

Parameters: fname (str) – The filename to open. mode (('r', 'w')) – The mode in which to open the file, either ‘r’ read or ‘w’ write

Examples

>>> from MDAnalysis.lib.formats.libmdaxdr import TRRFile
>>> with TRRFile('foo.trr') as f:
>>>     for frame in f:
>>>         print(frame.x)


Notes

This class can be pickled. The pickle will store filename, mode, current frame and offsets

calc_offsets()

read byte offsets from TRR file directly

close()

Close the open XDR file

Raises: IOError
offsets

get byte offsets from current xdr file

open()

Open a XDR file

If another XDR file is currently opened it will be closed

Parameters: fname (str) – The filename to open. mode (('r', 'w')) – The mode in which to open the file, either ‘r’ read or ‘w’ write IOError
read()

Read next frame in the TRR file

Returns: frame – namedtuple with frame information libmdaxdr.TRRFrame

TRRFrame(), XTCFile()

Raises: IOError
seek()

Seek to Frame.

Please note that this function will generate internal file offsets if they haven’t been set before. For large file this means the first seek can be very slow. Later seeks will be very fast.

Parameters: frame (int) – seek the file to given frame IOError
set_offsets()

set frame offsets

tell()

Get current frame

write()

write one frame into TRR file.

Parameters: xyz (array_like, shape=(n_atoms, 3), optional) – cartesion coordinates. Only written if not None. velocity (array_like, shape=(n_atoms, 3), optional) – cartesion velocities. Only written if not None. forces (array_like, shape=(n_atoms, 3), optional) – cartesion forces. Only written if not None. box (array_like, shape=(3, 3)) – Box vectors for this frame step (int) – current step number, 1 indexed time (float) – current time _lambda (float) – current lambda value natoms (int) – number of atoms in frame IOError
class MDAnalysis.lib.formats.libmdaxdr.XTCFile

File-like wrapper for gromacs XTC files.

This class can be similar to the normal file objects in python. The read() function will return a frame and all information in it instead of a single line. Additionally the context-manager protocoll is supported as well.

Parameters: fname (str) – The filename to open. mode (('r', 'w')) – The mode in which to open the file, either ‘r’ read or ‘w’ write

Examples

>>> from MDAnalysis.lib.formats.libmdaxdr import XTCFile
>>> with XTCFile('foo.trr') as f:
>>>     for frame in f:
>>>         print(frame.x)


Notes

This class can be pickled. The pickle will store filename, mode, current frame and offsets

calc_offsets()

Calculate offsets from XTC file directly

close()

Close the open XDR file

Raises: IOError
offsets

get byte offsets from current xdr file

open()

Open a XDR file

If another XDR file is currently opened it will be closed

Parameters: fname (str) – The filename to open. mode (('r', 'w')) – The mode in which to open the file, either ‘r’ read or ‘w’ write IOError
read()

Read next frame in the XTC file

Returns: frame – namedtuple with frame information libmdaxdr.XTCFrame

XTCFrame(), TRRFile()

Raises: IOError
seek()

Seek to Frame.

Please note that this function will generate internal file offsets if they haven’t been set before. For large file this means the first seek can be very slow. Later seeks will be very fast.

Parameters: frame (int) – seek the file to given frame IOError
set_offsets()

set frame offsets

tell()

Get current frame

write()

write one frame to the XTC file

Parameters: xyz (array_like, shape=(n_atoms, 3)) – cartesion coordinates box (array_like, shape=(3, 3)) – Box vectors for this frame step (int) – current step number, 1 indexed time (float) – current time precision (float (optional)) – precision of saved trajectory, see Notes

Notes

Gromacs specifies the precision a little bit strange. The coordinates will be multiplied by precision and then converted to an integer. This means a precision of 1000 yields an accuracy of 3 decimal places.

Raises: IOError