The euphonic-powder-map program can be used to sample
spherically-averaged properties from force constants data over a range
of |q|. The results are plotted as a 2-dimensional map in (|q|, \omega).
For example, to plot a coherent neutron-weighted powder spectrum from CASTEP force constants over a recommended |q| range, one could run:
euphonic-powder-map quartz.castep_bin --weighting coherent --energy-broadening 1.5
Note the text boxes below the intensity map, which allow the intensity limits of the plot to be adjusted without having to re-run the expensive powder calculation. To plot a DOS-weighted intensity from Phonopy force constants over a specified q range with denser sampling, in THz and with the intensity widget disabled:
euphonic-powder-map NaCl/phonopy.yaml --weighting dos --energy-unit THz --energy-broadening 0.15 --q-min 0.01 --q-max 4. --q-spacing 0.01 --no-widgets
To see all the command line options, run:
euphonic-powder-map -hYou can also see the available command line options at the bottom of this page. For information on advanced plot styling, see :ref:`styling`.
Sampling many q-points can be computationally expensive, so a progress bar will automatically be displayed if tqdm is installed
Inelastic neutron-scattering measurements have an accessible
(\mathbf{q}, \omega) range depending on the instrument
geometry: either the incident energy is fixed (direct geometry) or the
final energy is fixed (indirect geometry), and the scattering angles
are determined by reflector/analyzer/detector positions. The remaining degrees
of freedom (e.g. time-of-flight) are used to determine the
(\mathbf{q}, \omega) transfer. To simulate these constraints,
the parameter --e-incident or --e-final can be used to set the
constrained energy, and --angle-range can be used to fix the
detector range. So, for example, to simulate the MARI instrument at
ISIS with a 60 meV incident energy:
euphonic-powder-map quartz.castep_bin --angle-range 3 135 --e-incident 60 --q-max 11 --energy-unit meV --weights coherent
Spherical averaging is performed in a series of constant-q shells. The
--npts, --npts-density, --npts-min and --npts-max
options control the number of samples in each shell, while the
--sampling and --jitter options control the sampling scheme.
The :ref:`euphonic-show-sampling <sampling-script>` tool can be used
to visualise different sampling schemes.
While the default scheme is recommended for all production
calculations, it is generally necessary to tune the NPTS parameters.
While --npts sets a constant number of samples for each shell,
--npts-density sets the number of samples at a
1/LENGTH_UNIT-radius sphere, and applies quadratic scaling for other
distances. This may lead to inappropriately small or large numbers of
samples at low or high |q|, so the range is limited by
--npts-min and --npts-max. The program will print "Final
npts:" with the number of samples used at the largest sampling
sphere. If this is equal to --npts-max then the upper limit is in
use; you may wish to experiment with reducing --npts-density or
increasing --npts-max in such cases.
The --save-json option can be used to output the produced
:ref:`Spectrum2D` object as a Euphonic .json file with a specified
name for further use in Euphonic or other programs.
The Brille library allows linear interpolation of phonon frequencies and eigenvectors, as opposed to the Fourier interpolation used by Euphonic.
Linear interpolation should be faster, but less accurate, than Fourier interpolation, so can provide a performance improvement of the euphonic-powder-map script.
To use Brille with this script use the --use-brille argument, which by default will create a Brille trellis grid with approximately 5000 points from which to perform linear interpolation.
The grid type and number or number density of points on the grid can be changed using the --brille-grid-type and --brille-npts or --brille-npts-density arguments.
For more information on how Brille works with Euphonic and help choosing the number of grid points, see :ref:`Brille Interpolator <brille-interpolator>` and :ref:`euphonic-brille-convergence <brille-convergence-script>`.
.. argparse::
:module: euphonic.cli.powder_map
:func: get_parser
:prog: euphonic-powder-map