The `MCViNE_Phonon_CoherentInelastic_PolyXtal_process` Component

Union process: Coherent one-phonon scattering from a powder, using a full phonon dispersion (energies + polarizations) on a grid.

Identification

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

Description

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)

Input parameters

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' ([AA] cartesian, [amu], [fm], [barn], [barn]), same order as in the IDF files	0
T	K	Temperature	300
dw_core	AA^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	AA^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).	""

AT (	,	,		) RELATIVE
ROTATED (	,	,		) RELATIVE

The MCViNE_Phonon_CoherentInelastic_PolyXtal_process Component

Identification

Description

Input parameters

Links

The `MCViNE_Phonon_CoherentInelastic_PolyXtal_process` Component