def _bands_from_force_constants(data: ForceConstants,
q_distance: Quantity,
insert_gamma: bool = True,
frequencies_only: bool = False,
**calc_modes_kwargs
) -> Tuple[Union[QpointPhononModes,
QpointFrequencies],
XTickLabels, SplitArgs]:
structure = data.crystal.to_spglib_cell()
bandpath = seekpath.get_explicit_k_path(
structure,
reference_distance=q_distance.to('1 / angstrom').magnitude)
if insert_gamma:
_insert_gamma(bandpath)
x_tick_labels = _get_tick_labels(bandpath)
split_args = {'indices': _get_break_points(bandpath)}
print(
"Computing phonon modes: {n_modes} modes across {n_qpts} q-points"
.format(n_modes=(data.crystal.n_atoms * 3),
n_qpts=len(bandpath["explicit_kpoints_rel"])))
qpts = bandpath["explicit_kpoints_rel"]
if frequencies_only:
modes = data.calculate_qpoint_frequencies(qpts,
reduce_qpts=False,
**calc_modes_kwargs)
else:
modes = data.calculate_qpoint_phonon_modes(qpts,
reduce_qpts=False,
**calc_modes_kwargs)
return modes, x_tick_labels, split_args
def _get_debye_waller(temperature: Quantity,
fc: ForceConstants,
grid: Optional[Sequence[int]] = None,
grid_spacing: Quantity = 0.1 * ureg('1/angstrom'),
**calc_modes_kwargs
) -> DebyeWaller:
"""Generate Debye-Waller data from force constants and grid specification
"""
mp_grid_spec = _grid_spec_from_args(fc.crystal, grid=grid,
grid_spacing=grid_spacing)
print("Calculating Debye-Waller factor on {} q-point grid"
.format(' x '.join(map(str, mp_grid_spec))))
dw_phonons = fc.calculate_qpoint_phonon_modes(
euphonic.util.mp_grid(mp_grid_spec),
**calc_modes_kwargs)
return dw_phonons.calculate_debye_waller(temperature)
def sample_sphere_structure_factor(
fc: ForceConstants,
mod_q: Quantity,
dw: DebyeWaller = None,
dw_spacing: Quantity = 0.025 * ureg('1/angstrom'),
temperature: Optional[Quantity] = 273. * ureg['K'],
sampling: str = 'golden',
npts: int = 1000, jitter: bool = False,
energy_bins: Quantity = None,
scattering_lengths: Union[dict, str] = 'Sears1992',
**calc_modes_args
) -> Spectrum1D:
"""Sample structure factor, averaging over a sphere of constant |q|
(Specifically, this is the one-phonon inelastic-scattering structure
factor as implemented in
QpointPhononModes.calculate_structure_factor().)
Parameters
----------
fc
Force constant data for system
mod_q
scalar radius of sphere from which vector q samples are taken
dw
Debye-Waller exponent used for evaluation of scattering
function. If not provided, this is generated automatically over
Monkhorst-Pack q-point mesh determined by ``dw_spacing``.
dw_spacing
Maximum distance between q-points in automatic q-point mesh (if used)
for Debye-Waller calculation.
temperature
Temperature for Debye-Waller calculation. If both temperature and dw
are set to None, Debye-Waller factor will be omitted.
sampling
Sphere-sampling scheme. (Case-insensitive) options are:
- 'golden':
Fibonnaci-like sampling that steps regularly along one
spherical coordinate while making irrational steps in
the other
- 'sphere-projected-grid':
Regular 2-D square mesh projected onto sphere. npts will
be distributed as evenly as possible (i.e. using twice
as many 'longitude' as 'lattitude' lines), rounding up
if necessary.
- 'spherical-polar-grid':
Mesh over regular subdivisions in spherical polar
coordinates. npts will be rounded up as necessary in
the same scheme as for sphere-projected-grid. 'Latitude'
lines are evenly-spaced in z
- 'spherical-polar-improved':
npts distributed as regularly as possible using
spherical polar coordinates: 'latitude' lines are
evenly-spaced in z and points are distributed among
these rings to obtain most even spacing possible.
- 'random-sphere':
Points are distributed randomly in unit square and
projected onto sphere.
npts
Number of samples. Note that some sampling methods have
constraints on valid values and will round up as
appropriate.
jitter
For non-random sampling schemes, apply an additional random
displacement to each point.
energy_bins
Preferred energy bin edges. If not provided, will setup 1000
bins (1001 bin edges) from 0 to 1.05 * [max energy]
scattering_lengths
Dict of neutron scattering lengths labelled by element. If a
string is provided, this selects coherent scattering lengths
from reference data by setting the 'label' argument of the
euphonic.util.get_reference_data() function.
**calc_modes_args
other keyword arguments (e.g. 'use_c') will be passed to
ForceConstants.calculate_qpoint_phonon_modes()
Returns
-------
Spectrum1D
"""
if isinstance(scattering_lengths, str):
scattering_lengths = get_reference_data(
physical_property='coherent_scattering_length',
collection=scattering_lengths) # type: dict
if temperature is not None:
if (dw is None):
dw_qpts = mp_grid(fc.crystal.get_mp_grid_spec(dw_spacing))
dw_phonons = fc.calculate_qpoint_phonon_modes(dw_qpts,
**calc_modes_args)
dw = dw_phonons.calculate_debye_waller(temperature
) # type: DebyeWaller
else:
if not np.isclose(dw.temperature.to('K').magnitude,
temperature.to('K').magnitude):
raise ValueError('Temperature argument is not consistent with '
'temperature stored in DebyeWaller object.')
qpts_cart = _get_qpts_sphere(npts, sampling=sampling, jitter=jitter
) * mod_q
qpts_frac = _qpts_cart_to_frac(qpts_cart, fc.crystal)
phonons = fc.calculate_qpoint_phonon_modes(qpts_frac, **calc_modes_args
) # type: QpointPhononModes
if energy_bins is None:
energy_bins = _get_default_bins(phonons)
s = phonons.calculate_structure_factor(
scattering_lengths=scattering_lengths, dw=dw)
return s.calculate_1d_average(energy_bins)
