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ForceConstants

The ForceConstants object contains the force constants, supercell, and crystal structure information required to calculate phonon frequencies and eigenvectors at any arbitrary q via Fourier interpolation.

Reading From CASTEP

The force constants matrix and other required information can be read from a .castep_bin or .check file with :py:meth:`ForceConstants.from_castep <euphonic.force_constants.ForceConstants.from_castep>`:

from euphonic import ForceConstants

filename = 'quartz/quartz.castep_bin'
fc = ForceConstants.from_castep(filename)

By default CASTEP may not write the force constants, if you receive an error saying the force constants could not be read, in the .param file ensure a PHONON_FINE_METHOD has been chosen e.g. PHONON_FINE_METHOD: interpolate, and set PHONON_WRITE_FORCE_CONSTANTS: true, then rerun CASTEP to trigger the force constants to be written.

Reading From Phonopy

When using Phonopy with Euphonic, it is recommended that all the required data (force constants, crystal structure, born charges if applicable) be collected in a single phonopy.yaml file. This can be done by running Phonopy with the --include-all flag or with INCLUDE_ALL = .TRUE. (phonopy >= 2.5.0 only).

Required information is read from Phonopy output files using :py:meth:`ForceConstants.from_phonopy <euphonic.force_constants.ForceConstants.from_phonopy>`. A path keyword argument can be supplied (if the files are in another directory), and by default phonopy.yaml is read, but the filename can be changed with the summary_name keyword argument:

from euphonic import ForceConstants

fc = ForceConstants.from_phonopy(path='NaCl',
                                 summary_name='phonopy_fc.yaml')

If you are using an older version of Phonopy, the force constants and born charges can also be read from Phonopy plaintext or hdf5 files by specifying the fc_name and born_name keyword arguments:

from euphonic import ForceConstants

fc = ForceConstants.from_phonopy(path='NaCl',
                                 fc_name='force_constants.hdf5',
                                 born_name='BORN')

Calculating phonon frequencies and eigenvectors

Phonon frequencies and eigenvectors are calculated using :py:meth:`ForceConstants.calculate_qpoint_phonon_modes <euphonic.force_constants.ForceConstants.calculate_qpoint_phonon_modes>` (see the docstring for algorithm details). A Numpy array of q-points of shape (n_qpts, 3) must be provided, and a :ref:`QpointPhononModes<qpoint-phonon-modes>` object is returned. A recommended q-point path for plotting bandstructures can be generated using seekpath:

import seekpath
import numpy as np
from euphonic import ForceConstants

# Read quartz data from quartz.castep_bin
filename = 'quartz/quartz.castep_bin'
fc = ForceConstants.from_castep(filename)

# Generate a recommended q-point path using seekpath
cell = fc.crystal.to_spglib_cell()
qpts = seekpath.get_explicit_k_path(cell)["explicit_kpoints_rel"]

# Calculate frequencies/eigenvectors
phonons = fc.calculate_qpoint_phonon_modes(qpts, asr='reciprocal')

Calculating phonon frequencies only

This uses the same algorithm as for calculating both the frequencies and eigenvectors, only with lower memory requirements as the eigenvectors are not stored. This is done using :py:meth:`ForceConstants.calculate_qpoint_frequencies <euphonic.force_constants.ForceConstants.calculate_qpoint_frequencies>` which returns a :ref:`QpointFrequencies<qpoint-frequencies>` object.

Docstring

.. autoclass:: euphonic.force_constants.ForceConstants
   :members:
   :exclude-members: force_constants, born, dielectric