Eval15 C-vector

The c-vector is the normal of the reflection plane. The c-vector is defined in the laboratory (xyz) system. The c-vector has to be rotated around the rotation axis (in this figure the z-axis) to bring it into diffracting position. This position is reached when the angle between the incoming beam (the x-axis) and the c-vector is 90-θ (Braggs Law).

            sin(θ)
  cos(ζ) = ———————
            cos(χ)

with χ the inclination of the cvector relative to the plane perpendicular to the rotation axis (in this figure the xy-plane). Note: |cos(ζ)| should be less or equal to 1.0

sin(θ)=cos(χ)
So ζ = 0. Only one solution.
| sin(θ) | > | cos(χ) |
No solutions. It is impossible to bring this vector into diffracting position. These are so called cusp-reflections.
| sin(θ) | < | cos(χ) |
Two solutions. The c-vector will diffract if the rotation angle is set to ζ (right impact) or to -ζ (left impact).

C₀ is the c-vector before any rotation. The required rotation angle ω=ζ-φ (for right impact; -ζ-φ for left impact) will bring C₀ into the diffracting position C.

Set c-vector

There are 4 ways to set a c-vector:

set reflection indices
Use the hkl command to set the reflection indices. The c-vector will be calculated from the orientation matrix at the goniostat position. You can define the orientation matrix with the rmat command. If you do not define the matrix, eval15 will calculate one from the cell parameters (defined with the cell command). The default cell parameters are 10 10 10 90 90 90. Use goniostat to define the goniostat position.
You also have to set left or right.
set impact coordinates
Use the hor and ver commands to set the impact position. The value of left or right will be set. The walk command also generates c-vectors from impact coordinates. The initial rotation angle (φ in the figure) will be set to -ζ or +ζ (for left and right impacts respectively) resulting in a final rotation ω of 0 degrees (set cvectorrot to on to change this behaviour).
set θ, χ and φ
Use the commands theta, chi and phi to define the c-vector. You also have to set left or right.
select reflection number
Use boxopen, next or file to load a reflection from a boxfile.

Animation

Random

During animate) random c-vectors will be generated if randomcvector=on.
θ will be set to a random value between thmin and thmax. Chi will be set to a random value between -90 and +90°
If θ + abs(χ) + mosaic + animos exceeds the value of thetachilimit (default 90°) new values of θ and χ will be tried. Then φ will get a value between 0 and 180°
50 % of the reflections will impact left and 50 % will impact right.

Walk

With the walk command, c-vectors will be generated from impact positions. The impact positions are generated by stepping over the detector (with stepsize set by step).

Eval15
Eval15 commands