Union process: Coherent one-phonon scattering from a powder, using a full phonon dispersion (energies + polarizations) on a grid.
Author: Fahima Islam (McStas port of the MCViNE phonon CoherentInelastic_PolyXtal kernel; MCViNE by J. Y. Y. Lin et al.)
Origin: MCViNE, https://github.com/mcvine/mcvine (mccomponents/lib/kernels/sample/phonon/CoherentInelastic_PolyXtal.cc)
Date: 2026-09-29
Powder average of the coherent one-phonon cross section, computed from phonon energies and polarization vectors tabulated on a grid over one reciprocal cell (MCViNE IDF format, e.g. from phonopy via MCViNE tools). A random branch and a random Q vector in a cube are drawn until the event is kinematically allowed; phonon creation/annihilation, Bose factor and Debye-Waller factor included. Scattering coefficient: total coherent cross section / Vc.
Union process. Part of the Union components: define this process, collect it into a material with Union_make_material (absorption is set there with my_absorption, the absorption inverse penetration depth at 2200 m/s), assign the material to Union_box/Union_cylinder/Union_sphere/Union_mesh geometries, and add a Union_master after them. Geometry, attenuation and multiple scattering are handled by Union_master; this component provides the MCViNE kernel: the scattering coefficient and the final-state sampling (weight). Isotropic process (powder/liquid-like); rotation is irrelevant. The kernel code is shared with the standalone component MCViNE_Phonon_CoherentInelastic_PolyXtal (mcvine-lib.c). Uses share/mcvine-lib.h/.c and share/mcvine-union-lib.h/.c; the process type MCViNE is declared in share/union-lib.h and registered with Union_master in share/mcvine-union-lib.h.
Example: MCViNE_Phonon_CoherentInelastic_PolyXtal_process(idf_dir="MCViNE/fcc_toy_phonons", atoms="MCViNE/fcc_toy_atoms.dat", T=300, max_omega=45)
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
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idf_dir |
str |
Directory with MCViNE IDF phonon files: Qgridinfo, Omega2, Polarizations[, DOS] |
0 |
|
atoms |
str |
Atoms file: one row per atom ’x y z mass b_coh sigma_inc sigma_abs’ ([Å] cartesian, [amu], [fm], [barn], [barn]), same order as in the IDF files |
0 |
|
T |
K |
Temperature |
300 |
|
dw_core |
Å\(^{2}\) |
Debye-Waller core; <0: computed from the DOS (idf_dir/DOS or dos) |
-1 |
|
dos |
str |
Optional DOS file for the Debye-Waller factor (default: idf_dir/DOS) |
0 |
|
Vc |
Å\(^{3}\) |
Unit cell volume; 0: (2pi)^3/|b1.(b2 x b3)| from Qgridinfo |
0 |
|
max_omega |
meV |
Maximum phonon energy |
50 |
|
min_omega |
meV |
Phonons below this energy are skipped |
0.01 |
|
unbiased |
1 |
0: MCViNE sampling (rejection + empirical accessible reciprocal volume, a few % high in tests). 1: single Q sample in the cube with its exact volume, unbiased |
0 |
|
packing_factor |
1 |
Packing factor (scales the scattering coefficient) |
1 |
|
interact_fraction |
1 |
Union: fraction of interactions forced to this process (-1: by cross section) |
-1 |
|
init |
string |
Deprecated and unused, accepted like on the other Union components (see Union_init). |
"" |
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Component source code found in file MCViNE_Phonon_CoherentInelastic_PolyXtal_process.comp.
MCViNE documentation: https://mcvine.github.io