TEMStrain    

Dynamic simulation

The dynamic simulation (called from the main interface) is based on the Bloch wave scheme. The mean crystal potential (which is too small to influence results) is ignored. 

There are two ways to calculate atomic form factors. They are referred to as A and B. The method can be selected on the “Unit cell” tab of “Settings”. 

The method A utilizes the table of D.Waasmaier & A.Kirfel (Acta Cryst. A51, 416-431 (1995)) for calculation of X-ray relativistic form factors and Mott formula for structure factor calculations. For some materials, Debye-Waller  B-factors are calculated based on the following papers:
* H.X.Gao and L.-M.Peng, (Acta Cryst.  A55, 926-932 (1999) - Elemental 1, Elemental 2 and  Compounds 1 (crystals with zinc blend structure)),
* H.X.Gao, L.-M. Peng and J.M. Zuo,  (Acta Cryst.  A55, 1014-1025 (1999) - rigid-shell model - Compounds 1 (crystals with sodium chloride structure)),
* H.X.Gao and L.-M.Peng, (Acta Cryst.  A56, 519-524 (2000) - 11-parameter shell model - Compounds 1 (crystals with caesium chloride structure)),
* N.M. Butt and J. Bashir, (Acta Cryst.  A44, 396-398 (1988) - Elemental 3 - 293K),
* N.M. Butt, J. Bashir and M. Nasir Khan,  (Acta Cryst.  A49, 171-174 (1993) - Compounds 2 - 293K),
* N. Gopi Krishna and D.B. Sirdeshmukh, (Acta Cryst.  A54, 513-514 (1998) - Elemental 4 - 295±3K),
* V.F.Sears & S.A.Shelley, (Acta Cryst. A47, 441-446 (1991) - Elemental 5).
For compounds, mass-weighted average of the cation and anion factors is given. 

In the method B,  absorptive form factors are calculated based on the subroutine FSCATT of  A.Weickenmeier and H.Kohl (Acta Cryst. A47,  590 - 597 (1991)).  To use this approach,  the displacement Sqrt[<u^2>] must be provided for each atom; <u^2> denotes mean square thermal displacement at a given temperature.

The simulation is controlled by the entries of “Settings/Simulation/Dynamic simulation”. The first entry is a limit on “Maximal Miller index”. The number of beams involved is controlled by either “Minimal intensity ratio” or “Number of reflections”. For a given reflection, “intensity ratio” is the ratio of its intensity to the intensity of most intense reflection; reflections with intensity ratios above “Minimal intensity ratio” are taken into account. With the “Number of reflections” equal to N, the N most intense reflections influencing the pattern are taken into account. All three limits are applied, i.e. the set of displayed reflections is an intersection of  three sets determined by the above conditions. For eliminating one or two conditions, enter extreme values in the corresponding text box. The number N includes some reflections outside the screen which may also influence the intensity map; the margin for such reflections is small (about 50 pixels on each side). 

The units of “Foil thickness” are [A]. This parameter is applicable to all patterns of a project. Its value for a particular pattern is calculated by the program based on tilt angles.