The `MCViNE_LorentzianBroadened_E_Q_process` Component

Union process: Isotropic dispersion E(Q) broadened by a Lorentzian of Q-dependent width.

Identification

Author: Fahima Islam (McStas port of the MCViNE LorentzianBroadened_E_Q_Kernel; MCViNE by J. Y. Y. Lin et al.)
Origin: MCViNE, https://github.com/mcvine/mcvine (mccomponents/lib/kernels/sample/LorentzianBroadened_E_Q_Kernel.icc)
Date: 2026-09-29

Description

S(Q,E) = S(Q) L(E - E(Q); gamma(Q)) with L a Lorentzian of half width at half
maximum gamma(Q).
Q is sampled uniformly in [Qmin,Qmax]; the energy offset is sampled from the
line shape (up to 100 attempts). Neutrons with Ei below min(E(Q)-3w(Q)) do not
scatter.

Functions are given as strings evaluated at run time (MCViNE uses fparser):
+ - * / ^ (or **), % , comparisons, && ||, if(c,a,b), sin cos tan asin acos
atan sinh cosh tanh exp log log10 log2 sqrt abs floor ceil int sign cbrt,
pow atan2 min max hypot fmod, constants pi and e.

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_LorentzianBroadened_E_Q (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_LorentzianBroadened_E_Q_process(E_Q="20*sin(Q*1.5)^2", S_Q="1", gamma_Q="0.5", Qmin=0, Qmax=10, scattering_coefficient=10)

Input parameters

Parameters in boldface are required; the others are optional.

Name	Unit	Description	Default
E_Q	str	E(Q) expression [meV], variable Q [AA^-1]	"10"
S_Q	str	S(Q) expression	"1"
gamma_Q	str	Lorentzian HWHM gamma(Q) [meV]	"1"
Qmin	AA^-1	Lower Q bound	0
Qmax	AA^-1	Upper Q bound	10
unbiased	1	0: MCViNE sampling (retries without acceptance correction: over-weights when part of the Q range is forbidden). 1: single attempt, unbiased	0
scattering_coefficient	m^-1	Scattering coefficient (inverse penetration depth for scattering)	0
sigma_scat	barn	Alternative: scattering cross section per unit cell (used when Vc>0)	0
Vc	AA^3	Unit cell volume; when >0 the coefficient is sigma_scat/Vc	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_LorentzianBroadened_E_Q_process Component

Identification

Description

Input parameters

Links

The `MCViNE_LorentzianBroadened_E_Q_process` Component