[ Identification | Description | Input parameters | Links ]

The MCViNE_Phonon_CoherentInelastic_SingleXtal_process Component

Union process: Coherent one-phonon scattering from a single crystal, using a full phonon dispersion on a grid.

Identification

• 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

Description

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)

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
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).	""

AT (	,	,		) RELATIVE		Generate to clipboard
ROTATED (	,	,		) RELATIVE

Links

• Source code for MCViNE_Phonon_CoherentInelastic_SingleXtal_process.comp.
• MCViNE documentation: https://mcvine.github.io

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[ Identification | Description | Examples | Input parameters | Links ]

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Last Modified: Tuesday, 06-Oct-2026 21:19:12 CEST