This parameter targets systems with very large strains, like da/a > 0.1.

In the Bragg diffraction, it is convenient to express the Bragg deviations

*alpha=[(k+h) ^{2}-k^{2}]/k^{2}*

in the units of the Bragg peak FWHM that has the same order of magnitude as the
crystal susceptibility |*x*_{h}|. Then, the Bragg reflectivity of
a layer is ~1/*alpha*^{2}. Given that for a typical Bragg reflections
|*x*_{h}|~10^{-6} and the da/a parameter contributes to
*alpha* linearly, then the strain of da/a~0.1 corresponds to very large
*alpha*~10^{5}|*x*_{h}|, or the reflectivity of
~10^{-10}. On one hand, such small intensities may be neglected, and
on the other hand, if trying to take them into account, one may face numerical
errors in the calculations at large *alpha* since the *alpha* parameter
enters into some elements of the scattering matrices while their other elements
are constituted by the *x*_{h} only.

Let us consider a simple bi-layer system presented on Fig.1.

Such a system gives the Bragg curve as presented on Fig.2, where the parameters of
calculations are: t=5000A, wave=1.54A, code=Silicon, hkl=(111),
da/a = -0.5 = -0.625*|*x*_{h}|*10^{5}.

In this example the Bragg peaks from the layer and the substrate are well separated and when the incident X-rays are tuned to the substrate peak, the reflection from the layer may be neglected, i.e. the layer may be treated as amorphous, and vice versa in the vicinity of the layer Bragg peak.

The dynamical cutoff is handled by the parameter **alpha_max**. If at a particular
point of the Bragg curve the Bragg deviation |*alpha*| exceeds **alpha_max**,
the layer is dynamically "converted" into "amorphous". However, if all the
layers in the target system happen to be far from their Bragg condition, the
threshold is not enforced on the layer(s) with the minimum |alpha| among all
the layers. This avoids zero intensity at the calculations of wide-range
Bragg curves from an unstrained single crystal. In all cases any manipulations
with layers are reported in the calculations log file (the TBL file).

Normally the **alpha_max** parameter does not need be touched. One only
needs to tweak it when the calculation fails with the error message like:

GID_SLm: the job was aborted due to numerical errors detected while calculating reflection coefficient(s). This may be caused by precision loss because of weak reflection (check the order of xh!) or large deviations from the Bragg condition. Check GIDxxxxx.TBL for more details.

The dynamical cutoff must be used with care on the systems with the gradient of
da/a from layer to layer. In such cases an important interference between the
layers with close da/a values might be lost. In the case when the cutoff is
applied to a gradient system, it is recommended to repeat the calculations
using different values of **alpha_max** in order to study the effect of
the approximation.

It is not advised (although possible) to use **alpha_max** <
10^{3} since the program should not suffer from precision losses
in this *alpha* range.

Please also note that calculating the Bragg curves at very large *alpha*
when the reflectivity is below |*x*_{h}|^{2} violates the
two-wave diffraction approximation and the results of such calculations must
be treated with big precaution! This point is well illustrated by the following
picture demonstrating that the right tail of the above calculated 111 Bragg
curve overlaps with the 333 reflection from the same sample (the yellow line).

It is obvious from this example that the two-wave approximation is not applicable around the incidence angle of 44 degr. and at higher angles using the 111 reflection in the calculations is incorrect at all.