Union process: Coherent one-phonon scattering from a single crystal, using a full phonon dispersion on a grid.
Author: Fahima Islam (McStas port of the MCViNE phonon CoherentInelastic_SingleXtal kernel; MCViNE by J. Y. Y. Lin et al.)
Origin: MCViNE, https://github.com/mcvine/mcvine (mccomponents/lib/kernels/sample/phonon/CoherentInelastic_SingleXtal.cc)
Date: 2026-09-29
Single-crystal coherent one-phonon scattering with dispersion and polarization vectors from MCViNE IDF files (periodic over the reciprocal cell b1,b2,b3 given in Qgridinfo; orient the crystal by rotating the component). For a final direction (4pi, or a cone towards a target when focus_r>0) and a random branch among those with Emin < 1.5 Ei, all final speeds solving omega(Q) = |Ei-Ef| are found (Ridders’ method on nsteps sub-intervals of [0,2vi]); one is chosen and weighted by the delta-function Jacobian, |sum_d b_d/sqrt(M_d) exp(iQ.d)(Q.e_d)|^2, Bose and Debye-Waller factors.
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). Orientation: this process is anisotropic; its frame (Q vectors, reciprocal vectors, atom positions) follows the ROTATED placement of this component. The kernel code is shared with the standalone component MCViNE_Phonon_CoherentInelastic_SingleXtal (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_SingleXtal_process(idf_dir="MCViNE/fcc_toy_phonons", atoms="MCViNE/fcc_toy_atoms.dat", T=300)
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
|
|
|||
|
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 |
|
deltaV_Jacobi |
1 |
Relative velocity step for the numerical Jacobian |
0.001 |
|
nsteps |
1 |
Sub-intervals for the root search in vf |
100 |
|
xacc |
m/s |
Root accuracy in vf |
10 |
|
unbiased |
1 |
0: MCViNE sampling (retries up to 100 direction/branch draws until omega(Q)=|Ei-Ef| has a solution, without correcting the weight: over-estimates when many directions have no solution). 1: single attempt, 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). |
"" |
|
|
|||
|
|
|||
|
|
|||
Component source code found in file MCViNE_Phonon_CoherentInelastic_SingleXtal_process.comp.
MCViNE documentation: https://mcvine.github.io